Welcome to the forum! Sharing your story is the first step towards finding a solution.
Humans are complex. Everyone is unique (I know I am, haha - seriously everyone is unique), and that's what keeps life interesting. More importantly, we are all dynamic, i.e. we think, feel and act differently at different times, days, depending on a multitude of factors.
Firstly, acting in a certain way does not mean that someone must have a disorder. The diagnosis of a disorder requires more rigorous analysis - factors include:
-length of period in which the behaviour has been observed;
-extent of the effect of this behaviour on the individual's life, etc.
Normally, all people may feel sad at times for a day or two every once in a while, but that doesn't mean we are all clinically depressed. That said, your partner may be totally mentally healthy (.. he may have a toe that he hurt in the last footy match) - he may have the disorder you talked about, or a disorder we're not aware of. So I wouldn't jump to conclusions.
Secondly, the interaction between two people is very complex as well! Think of each person carrying a large hidden bag of thoughts and feelings that depend on what they had experienced on that day (say, little sleep, or a really yummy coffee, or received a compliment from his/her manager at work, or their rightmost toe is hurting from the last footy match), or what they are about to do (say they're about to give a big presentation to a 100 people! or about to go sleep, or about to enter a new stage in life, that may be a new degree at uni, or whatever). Different people exhibit those thoughts and feelings differently - some through their body language more obviously than others, and some exhibit them differently depending on who's in the room. So maybe you can utilise those to understand why he behaves in a certain way from time-to-time. What pattern of events, etc. happen before such behaviour results? What conversations were taking place? What is happening in his/your life/work, etc.? If you find a pattern, when you see an early sign of it, do something super positive that makes him happy/comfortable/clear his doubts (and be creative about it - surprise him!). A hug or the like usually works well.
Also - maybe your partner needs a little bit more space to re-gather and recharge himself. Most partners come back with a lot more love after having their space.
With the little info given, this is my best advice. Hope you find a solution.