For Fun And For Keeps?

When in a relationship do you decide if it's for keeps or just for fun? I've always dated by the rule that it's one or the other. Meaning, it's either a fling, or understood that it's not really serious, or it's serious, and it could become something, a major something.

So when do you know and how do you decide? There are "those" people that say you just know when you know. Then there are other people that say it's something you learn and decide over time. Then there's me. I think if after a certain amount of time, you don't have fairly strong feelings for someone, that person doesn't treat you the way you want to be treated, and you can't really see yourself with that person if someone hotter, sexier, etc. comes along, then even though it might have started out for keeps, it's really just for fun. Better to get out now before feelings are hurt and lines are drawn. I've always thought this was how it worked, could I have been wrong about this all along?

I've started to wonder lately because I'm currently involved in something that is really fun, and I'm really enjoying it. However, it's not in a fling sort of way. We spend a lot of time together, and we have been "coupled" by our friends. So, could it be that when you find someone that is for keeps, it's also really fun? Huuuum, just when you think you've got something figured out, someone fun comes along that you might want to keep.