I used to rarely (read, never) stand while climbing. This is partly because I can spin uphill really efficiently, but also because I never felt stable out of the saddle on climbs.

Then, I bought a new bike, and without thinking about it I am climbing out of the saddle (and sprinting out of the saddle) so much more than I used to.

I never would have guessed, but I think frame geometry plays a big role here. I don't think it is about bike fit because my old bike fits me like a glove (arguably better than the new one).