can someone tell me why?

Ar
Is it because we are Young? He's gonna be 18 in may.. when he first found out I was pregnant he ran off... but then he came back and when he told his parents and they he were happy he seemed so happy about it and told me he wanted to have a life together. And today he left again. We haven't fought in over a month in a half, until like 3 days ago but he leaves everytime it gets hard. Should I just let him go? Like honestly.. we are having a baby together but he always stresses me out. Please don't judge or say anything rude i just want guidance.. that's all I'm asking for.