So... as things are currently running, the only reason why Starscream joined the Decepticons was because of Shrapnel.

The trouble with writing Decepticon report cards is that Starscream is likely to give some of them good ratings, and it's hard to think of things that are both positive and funny. Starscream insulting people is so much more fun!

I think we all know what finally won him over, though:

Megatron: Megatron is the most violent, selfish and ill-mannered mech it has ever been my displeasure to interface with. I must have him again immediately.