Whilst I agree with Deadlift, having fun is the most important part of the game, I do feel that we as gamers have a responsibility to help our opponents have fun as well. If not out of wanting to share a fun game but also in a self-serving manner of ensuring we have people who are happy to play against us. This doesn't mean that we should all only play the worst lists, but instead we should try to play in the same tier (for want of a better word) as our opponent.
one of the reasons why I haven't commented on the OP's question is because I have no idea what the OP plays and fields. was the OP steam rolling every game to the point where the space marine player felt they needed to take more competitive options, did the OP's opponent just decided that netlisting was in his best interest.
I myself am certainly not a competitive player, nor do I really like a lot of the 40k fluff, but I do enjoy a close tactical game (within my self imposed limits) along with building converting and painting Miniatures.