Basic: Roy and Riza are getting married, and Havoc's doing his best to fake being happy for them.
More In-Depth: The fake happiness is due to the fact that they've each used him for years without being aware of it, are continuing to do so, and he knows it.
Backstory: At first, it was Roy, who (like in many fics, it seems) was using him as a substitute for Hughes, at least until Hughes died. Then he just up and broke it off. Havoc knows exactly why though, knew from the beginning. Then it was Riza, who he admittedly had a bit of a crush on, who only accepted his affection because she more or less had given up on ever getting Roy's attention, and was willing to take second best. Finally, Havoc just can't take it anymore, and more or less throws them at each other, because he knows they'll be happy together, without him in the picture. Only, since he didn't know about the relationship with Riza, Roy asks him to be best man, again using him in place of Hughes. Sort of a vicious circle thing, and it just leaves Havoc more depressed and empty each time it goes around.
Voila. Please take this one, somebody. It needs a good home other than in my head.