It varies. It might be because that DPS has shown a good example and attitude. It might be that they've done their job well as far as I've seen. It might be they're new and they've put in a good effort and listened to mechanics or advice if they've needed any. It could be that the other people in the party I don't feel deserve my comm and that DPS was doing fine, it might be that the others were sloppy and the DPS weren't. If they first struggled, but persevered and learned from their mistakes, then I might comm them. If they've been especially social and helped make the experience more pleasant or enjoyable, then i might. It could be that they have my back. Or they do something that I think was impressive. It might be just that they've executing their rotations fine, sometimes I pay attention to what other players are doing, particularly when healing because you can pre-empt more when you're seeing what people are doing. Or everybody else has ran out before I got a chance to comm. If that DPS takes initiative where others might not have, then a comm.

I also try to actively avoid giving a tank or healer bias in comms. DPS can do good jobs too. If I'm ever not sure who to comm, then it'll be for completely arbitrary reasons, like their name or glamour or they're on a job I like or at random.