I not only do that with photogrpahy but with my car. I've put thousands into my car and am only going to put more into it and sometimes I ask myself...why? But then I remember how fun heel toeing from 4th to 3rd gear into a 25mph speed limit corner at 75mph is, then I remember... Still working on the turbo set up to pull myself OUT of the corner. Entrance and mid corner are the only fun part right now :-/
Same with photography. I see amazing photo's and just remember why I do it. I've always been an artisic person, and have always been into drawing. Ever since I took up photography, I've been able to find more beauty in everyday things and almost everyday, I see something that would make a good picture. That also makes me remeber why I do it...
Speaking of drawing, I havn't drawn in a while :-/
You'll do it with any hobbie you have, especially if you've been doing it a long time.