My boyfriend gets mad at me for little things, why?

i know he has problems with depression and such but he can be happy one minute and the next he will find something to be mad at me for. i dont have cell phone service so i can only call him using wifi and sometimes it acts up and we can't hear well, it also dropps the calls and he gets mad at me and won't talk to me for a while after, then he will text me like nothing happened. Does it just mean that he cares or holds me to a high standard or am i actually doing something wrong?
My boyfriend gets mad at me for little things, why?
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