Y do guys always accuse u of cheating when u not & wen u accuse dem they hav every reason y they not cheating.

I have a boyfriend that I hav been faithful 2 for the past year & he claims that he has been the same, but i am startin 2 have doubts because he is always accusin me and i sit home all day waitn for him to come back home and barely go out. He comes in lookin for stuff and wanting to argue almost every night. I beg and plead for him 2 belive me but he doesn't. So when i start to come at him with the same stuff he comes up with every possible reason why he isn't cheatin and how he is being faithful. I only accuse him when he accuse me.How can he expect me to believe him when he doesnt believe me. I wanna know why do guys do this and could there be a possiblity that he is cheatin on me and just tryin to watch ne so that i can't catch him in his wrongful acts? Should i continue to take this mess from him or should i be like **** it and just leave or is there any kind of way that we could try to work this out.