I've been 'chubby' since the 3rd grade. I'm 29. I got into shape once, 5 years ago, then fell off the wagon for various reasons.
its a challenge to myself to see what I can turn my body into. NOBODY on either side of my extended family has a great body, so I nominated myself to be the posterchild for 'the relative everyone gets jealous of at the family christmas gathering'.
plus it helps motivate my wife, who's always been heavier than me. and she in turn motivates me.
I think about my workout for hours in advance. I listen to music that pumps me up for 20-30 mins before I leave for the gym every day.
during cardio, I just veg out mentally and jam to music, and usually stare at the hottie on the stairmaster in front of me wearing the tight spandex...ok I got off topic a little there
I also cheat...I take 1g of l-tyrosine most days. its a non-essential amino acid that we can run low on during intense training. it has an effect on mood and mental focus. its not proven, but a lot of people take it to keep from mentally burning out.
a lot of fat burner supps have it too because its easier to mentally burnout on a lower calorie fat-cutting type diet.
its dirt cheap too, $6 for a big bottle.