for me, wanting to be bigger comes down to a few things, most of which have already been mentioned by different people here:
1) im competitive. i want to be the biggest any woman im with has had. this is the biggest reason, and like red said it comes down to power.
2) if you have a big dick, you know you have a big dick. good confidence boost and just generally makes you feel good about your assets.
3) once a long term partner gets used to a bigger size i beleive its more pleasurable (mainly girth)
4) just having a bigger dick feels better
5) the process of gaining in itself is rewarding (not unlike
weight lifting)