What bra size do you wear?
If you've got quite small boobs, it's not that unusual not to wear a bra, I don't think.
If they're quite big, you probably need to get measured again and get a well fitting bra. It shouldn't be uncomfortable or make your boobs look weird.
It's absolutely your choice if you wear one - If I didn't, I think the weight would make me uncomfortable after a while and I'd feel underdressed. I also dread to think of the attention that it would attract. And I'd probably worry that they'd sag, especially when I run etc. Your decision, though. If you're happier without (and have been for six years), stay without. If you're feeling unsure, try a soft, well-fitting bra and see how you go, but remember to give yourself a bit of time to get used to wearing one so it's a fair comparison!