People can always look good when they feel good. But their real character comes out when they feel bad!
When you are having the kind of day where even you know that you are hard to get along with, how does he respond to you? Does he have the patience and fortitude to hang in there with you to find out what the problem is? How about when he is feeling low? How does he relate with you? Does he just shuffle off into his shell? Does he say nasty things to you? Or does he rise above his moodiness and at least really make effort to show that you are very important to him?
What happens when you both are in conflict and in a bad mood? How do you argue? How do you resolve your arguments? You may not see these questions necessary now, but this is the point at which relationships get either deeper or shallower.
You think you want to get married? Grab the opportunity to discover his character, your character and the character of your relationship!
So instead of hiding out when you are feeling down, this may be a good opportunity to see if he is there for you for real- a real chance to build the relationship around something other than just a state of feeling good!