I don't think that having depression makes you either a Dom or Sub although it could add on to being more irrational while in the role of such. For instance a Dom could possibly take his/her frustrations on a Sub undeserving of punishment (at the moment). A Sub could possibly become too dependent too quickly on any Dom who shows him/her a touch of affection.
This is just my opinion, but I do believe that it is possible for someone with depression to take the role of a dom or sub simply to relieve burdens of their daily life. They could grow accustom to the lifestyle without having any attraction to it.
I definitely have to agree here. A Dom that either does not know his power or abuses it for selfish "therapeutic" reasons is very dangerous. If a Sub is very prone to depression or very dependent to begin with then a Dom could screw with their psyche. The gain for the Dom here would be for them to have unneeded control over their sub's daily feelings, actions, attitudes and overall self esteem. There is no trust here just a pretending power hungry Dom.
When finding a Dom try to look for one that has both of your interests in mind. Trust has to be built up, also remember that the sub gives the power away and can take it right back if it's abused.
I'm not sure if this helped or I'm only referring to a personal experience of a good friend of mine but good luck.