Help! I live with my grandpa and he gets really angry and offensive at times, other times he's really loving and happy. What does this mean?

He’s around 70 years old, he’s been like this for years but more recently it’s been back and forth between being really happy and really angry, he yells over dumb things like toilet paper being rolled down too much or a little crumbs. He also does things that he doesn’t need to do and then complains about doing them. He offends me and my other family memebers a lot. I’m only 19 I don’t know much about older people but I just want to know what’s going on with him so I know how to deal with it. Not gonna lie I argue back with him when he gets angry and starts argument, I try to avoid him all the time, and sometimes he really gets under my skin and makes me angry or really upset. I just wanna know what’s going on with him and how to deal with it