You can't explain it really, it's just something you KNOW. Sorta like being in love but totally different

I have a P&S at the moment, it lives in my pocket or bag. I love it and it goes a lot of places with me. But sometimes I just wanna scream because I just can't get the shot I want with it as the "manual" settings are very limited.
Like last week. There was a huge huge huge cheesey moon, absolutely beautiful. DF and I, with the dog, drove to the end of town to watch it. I took my P&S and my tripod. Nothing I did could get a shot of that moon. That frustrated the hell out of me.
That's just one example. I guess I'm trying to say that you will know you're ready for a DSLR when you feel the way I did on that night. Once it happens once, it's going to keep happening and then you're doomed
Though I'm still gonna keep my P&S as it's great to bring on a night out when there's a very good chance beer will get spilt on it!! It's surprisingly waterproof......