Because Lydia just said that he was attractive. And that she wanted to have his baby. And apparently this place really was paradise and OH MY GOD he was going to get to have sex with Lydia Martin. He felt a bit weak in the knees at that realization, but managed to stay on his feet this time.
There was no question that Stiles would dote on her as much as possible while she was pregnant. They may not have any creature comforts here, but he would do whatever he could to make her as comfortable as possible.