"Anger makes men stupid." [spoilers TWoW]

Stannis seems pretty confident in TWOW Theon I, likely because he's got a bit of a Davos-esque plan in mind to misdirect the approaching forces both by replicating false lighthouses and using firsthand knowledge of the real terrain, as per their last on-the-page conversation before Davos departs for White Harbor:

There's much I don't understand," Davos admitted. "I have never pretended elsewise. I know the seas and rivers, the shapes of the coasts, where the rocks and shoals lie. I know hidden coves where a boat can land unseen. And I know that a king protects his people, or he is no king at all" (Davos VI, ASOS)

He's got the measure of his foes, and he knows what he's doing.

"The snow had covered up the pits, so they rode right into them. Aenys broke his neck, I heard, but Ser Hosteen only lost a horse, more’s the pity. He will be angry now.”

Strangely, Stannis smiled. “Angry foes do not concern me. Anger makes men stupid, and Hosteen Frey was stupid to begin with, if half of what I have heard of him is true. Let him come.”

“He will.”

“Bolton has blundered,” the king declared. “All he had to do was sit inside his castle whilst we starved. Instead he has sent some portion of his strength forth to give us battle. His knights will be horsed, ours must fight afoot. His men will be well nourished, ours go into battle with empty bellies. It makes no matter. Ser Stupid, Lord Too-Fat, the Bastard, let them come. We hold the ground, and that I mean to turn to our advantage.”

Despite their dire circumstances things look like they might turn out better than you'd expect for the current occupants of the Crofters Village, right?

Maybe. But... Stannis is a man we know takes a very long time to process what's going on in his own head.

"Only Renly could vex me so with a piece of fruit. He brought his doom on himself with his treason, but I did love him, Davos. I know that now. I swear, I will go to my grave thinking of my brother's peach." (Davos II, ACOK)

And this was not his original preferred strategy, before word of Davos's death had reached him.

"The merman of Manderly was not amongst those banners Lady Melisandre saw in her fires," Jon said. "If you had White Harbor and Lord Wyman's knights …"

"If is a word for fools. We have had no word from Davos. It may be he never reached White Harbor. Arnolf Karstark writes that the storms have been fierce upon the narrow sea. Be that as it may. I have no time to grieve, nor wait upon the whims of Lord Too-Fat. I must consider White Harbor lost to me. Without a son of Winterfell to stand beside me, I can only hope to win the north by battle. That requires stealing a leaf from my brother's book. Not that Robert ever read one. I must deal my foes a mortal blow before they know that I am on them."

Jon realized that his words were wasted. Stannis would take the Dreadfort or die in the attempt. (Jon IV, ADWD)

There is no hope whatsoever now of dealing his foes a mortal blow "before they know that [he is] on them.", yet he's dragged his men onward step by step, through the worst snowstorm many of them have ever seen, though they're starving, freezing, and dying in droves. His plan for Winterfell is now the complete reverse of the tactic he planned to use for the Dreadfort: to rely on them knowing, rather than their suprise. Even though it seems as if the very terrain itself is trying to hold him back. So... what changed?

Stannis Baratheon paced the floor. The tower was a small one, dank and cramped. A few steps brought the king around to Theon. “How many men does Bolton have at Winterfell?”

"Five thousand. Six. More.” He gave the king a ghastly grin, all shattered teeth and splinters. “More than you.”

"How many of those is he like to send against us?”

"No more than half.” That was a guess, admittedly, but it felt right to him. Roose Bolton was not a man to blunder blindly out into the snow, map or no. He would hold his main strength in reserve, keep his best men with him, trust in Winterfell’s massive double wall. “The castle was too crowded. Men were at each other’s throats, the Manderlys and Freys especially. It’s them his lordship’s sent after you, the ones that he’s well rid of.”

“Wyman Manderly.” The king’s mouth twisted in contempt. “Lord Too-Fat-to-Sit-a-Horse. Too fat to come to me, yet he comes to Winterfell. Too fat to bend the knee and swear me his sword, yet now he wields that sword for Bolton. I sent my Onion Lord to treat with him, and Lord Too-Fat butchered him and mounted his head and hands on the walls of White Harbor for the Freys to gloat over. And the Freys… has the Red Wedding been forgotten?”

"The north remembers. The Red Wedding, Lady Hornwood’s fingers, the sack of Winterfell, Deepwood Motte and Torrhen’s Square, they remember all of it.” Bran and Rickon. They were only miller’s boys. “Frey and Manderly will never combine their strengths. They will come for you, but separately. Lord Ramsay will not be far behind them. He wants his bride back. He wants his Reek.” Theon’s laugh was half a titter, half a whimper. “Lord Ramsay is the one Your Grace should fear.”

Stannis bristled at that. “I defeated your uncle Victarion and his Iron Fleet off Fair Isle, the first time your father crowned himself. I held Storm’s End against the power of the Reach for a year, and took Dragonstone from the Targaryens. I smashed Mance Rayder at the Wall, though he had twenty times my numbers. Tell me, turncloak, what battles has the Bastard of Bolton ever won that I should fear him?”

You must not call him that! A wave of pain washed over Theon Greyjoy. He closed his eyes and grimaced. When he opened them again, he said, “You do not know him.”

"No more than he knows me.”

"Knows me,” cried one of the ravens the maester had left behind. It flapped its big black wings against the bars of its cage.

"Knows,” it cried again.

Stannis turned. “Stop that noise.”

Behind him, the door opened. The Karstarks had arrived. (Theon I, TWOW)

Stannis is really angry now he knows Davos is not lost, missing, or delayed, or even just brought down and drowned in a storm, but that Manderly himself (seemingly) killed him. And he doesn't seem to know it yet ("I have no time to grieve"). And without Davos actually there beside him there's nobody around to stop him "doing his duty" no matter the cost any more either. He's not listening to what he doesn't want to hear, only what he does. He's distracted, and no longer checking how solid the information he's given is—its actually the half-insane suicidal torture victim that's dangling from a wall that gets them back on topic! And neither of them have remembered that "Arya Stark" was what was keeping the North unified under Bolton's rule. And now, as far as they know, Stannis "I cut down weirwoods and burn people alive" Baratheon has kidnapped her.

Stannis believes this fight is winnable, that it's time to risk it all (or die trying). But he's paying attention to the wrong part of that last conversation he and Davos had too. Knowing the terrain, "where the rocks and shoals lie", is all well and good, but at the end of the day "a king protects his people, or he is no king at all". That's the important bit.

But just as he says: Anger makes men stupid.