TL;DR: 1) He didn't know he was charging into thousands of bullets. 2) Nobunaga tricked and forced his hand.

I talked about the myth and reality of Nagashino in depth here. Samurai archives went into even more depth here.

I also checked both the relevant passages of the Shinchoki and Shinchokoki. Samurai Podcast paraphrase Koyogunkan and Mikawamonogatari on top of that as well (unfortunately I couldn't find them online, but I trust Samurai Podcast).

Before the battle Nobunaga went to great pains to conceal his strength and deployment and make himself look weak. Shinchoki recorded his incredibly slow march onto the battlefield that made it look he was hesitant to engage. Nobunaga as the relieve army instead of marching to assault the siege lines around Nagashino parked his army 4km away in the hills and refused to move. Tokugawa Ieyasu was deployed in front during marching which likely acted as a screen for the larger Oda force. One of Tokugawa Ieyasu's retainer also promised to switch sides if Takeda Katsuyori attacked.

The rain on the couple of days before the battle also helped conceal the alliance deployment, numbers, and fieldwork construction. The alliance's field fortification network in depth also divided his forces into many defensive ranks to soak up brunt of each of the Takeda attack by having the troops pull back to the next line as soon as the Takeda were about to break through. To the attacking Takeda, they were easily breaking through successive lines of defenses, when in reality they were walking into a trap.

Finally, a big thing that made Katsuyori actually attack was the surprise attack on the siege lines by Sakai's forces at daybreak prior to the battle. It effectively relieved Nagashino, cut off Takeda Katsuyori's retreat, and pinned his army between two forces. At that point for Takeda Katsuyori, he could try go retreat north east, effectively losing anyway and be pursued and harassed all the way. Or he could attack a seemingly weak army with someone promise of mid-battle defection and possibly win. It was fight and have what seems like a good a chance of winning or retreat and take the losses and lose for sure (though not nearly as badly as it turned out, but he couldn't have known that).

As an aside, in my opinion Shingen's over-rated. But that's another topic.

EDIT: Also. No three rank volley fire. What was described in the not-as-accurate Shinchoki is physically impossible. Also there was highly unlikely to have been 3000 guns in the Oda army for the main battle, but the exact number of guns is relatively unimportant.

Yes of course lol battle of Temmokuzan I wonder why mitsuhide akechi decided to betray nobunaga I've read things like he was publicly abused and criticised and another said he was resentful due to being over looked for promotions but who knows maybe he was just a greedy disloyal general

Katsuyori was hoping on the rain but took to long and katsuyori got inpatient and ordered the charge the rain came after the slaughter