Why does he act this way, all the time?

His mood changes constantly. He gets cranky from the littlest things, and decides to treat me like crap, even if I haven't done anything to him. And he rarely ever wants to talk about things that are bothering him - I don't pester him about it either, unless it goes on for an extended period of time. (I know he works hard all day, 5-6 days a week, that's why I am there for him when he gets home, always)
Like today, I briefly talked to him on the phone, he was great, actually in a silly mood. When we went to get food for dinner, he was still in that goofy state, happy.
Then, in the blink of an eye, he's extremely cranky because he didn't have the right kind of dipping sauce... goes off by himself to sulk in his bad mood.
After a while, I go down to see him and I sit next to him to go on my computer, while he's watching TV. I was being silly, asking him what he's watching, joking around, etc. He pushed me away, responded nastily...
He got mad at me for being down there, for some reason, because he needs his dog to be out&about upstairs, therefore watched by someone( he was in his bed while we all were eating), but his parents were here... they could have watched him for a bit...
????

He does this kind of thing often, almost every day, it's not just a "male-PMS" thing. Even if I'm in a bad mood, I don't act this way towards him at all.
This hurts me because I just try to cheer him up and all he does is push me away and disrespect me, and it makes me not want to be around him if he's going to act like that.

What is your relationship status? How long have you been together? Are you married?

I'm only speculating of course, but it sounds like he may be unhappy with the relationship for some reason. You should ask him directly. Remember, relationships are a 2-way street. You have a right to be treated with respect. You have rights here. No one should control a relationship.

The first thing that pops to mind is: What were his developmental years like?

What type of environment did he grow up in? Was he physically, verbally or sexually abused as a child? How was he in school, from elementary to secondary, behaviorally and academically? Would you feel comfortable sharing a little bit so that we may all have some background information? I think doing so would allow for better responses.