That my boyfriend gets boners when we are alone a lot? :)

Okay so my situation is that my boyfriend gets boners almost every time we are alone together (his house / my house etc.) And i help him with them most of the time. But i dont know whats causing them. Its like when he really looks at me closely up it goes poking my arm hahaha. At first he was really embarrassed by them but now he seems to enjoy when he gets them. And they have been happening a lot these past few months. Is it normal? Guys does this happen to you? And is it normal that it turns me on when he gets them? Please let me know. :)