It could also be that they are used to Stiles being the one who takes care of them and remembers things for them. He's always been the planner and the one to keep things in order. So they unconsciously assume that he doesn't really care about his own birthday as much as he does about helping them, and they forget all about it.
He might be really hurt. Stiles is a sensitive guy deep down, and having his friends, who are like family to him, forget his birthday would sting. He could go into a bit of a sulk and isolate himself for a while.
Perhaps Stiles has found something else that he deems more important. It could be a new project, a relationship outside the pack, or a personal goal he wants to pursue. He might think that attending pack meetings is taking away time from this new focus in his life. Also, he could be facing some internal struggles, like self - doubt or a crisis of identity, and he needs some time away from the pack to figure things out.
Perhaps he feels that the other pack can offer him more opportunities for growth. In some fanfictions, the new pack might have access to different kinds of magic or knowledge that Stiles is eager to learn. So, he joins them to expand his skills and understanding.
One reason could be that he's tired of always being in the background. In the pack, others with more obvious powers might overshadow him. So he decides he's done and wants to find a place where he can shine on his own.
One possible reason could be that he feels he's a liability to the pack. Maybe he doesn't have the same supernatural abilities as the others and fears he'll hold them back during dangerous situations.
One way is for the pack members to stand up for Stiles more often. If someone outside the pack is giving him a hard time, they should be there to protect him. Also, they could share their secrets with him, which would show that they trust him. This trust is an important part of any relationship and would help Stiles realize that they don't actually hate him.
One reason could be a misunderstanding. Maybe someone in the pack misinterprets Stiles' actions as being against the pack's interests. For example, if he's involved in something that looks like it's endangering the pack but in reality he's trying to protect it.
In this kind of fanfiction, Stiles might decide to leave the pack due to feeling constantly underestimated or unappreciated. He could go off on his own and start to develop his own skills and relationships outside of the pack dynamic. Maybe he finds new allies who value him more than the pack did.
There could have been some misunderstandings. Perhaps Stiles overheard a conversation that he misinterpreted. Or some actions of the pack members were misconstrued by Stiles. For instance, if they were joking around but Stiles took it the wrong way and thought they were being mean on purpose, which made him think they hated him.
Well, Naruto could initially be shocked that no one remembers. But then he might try to play it cool, because that's his nature. He could also think that they might be planning a surprise later. However, as time passes and still no one remembers, he would start to feel really down. He might even go off on his own to brood about it, wondering if he's not as important to his friends as he thought.