There are a lot of great reasons to hire a personal trainer. I think the two I hear most often are:
1. It keeps things interesting
2. We don't let you quit just because you are "tired"
You push through a workout when you are tired or "just not feeling it". You do it because you KNOW you will feel better afterwards. You KNOW your clothes will fit better. And you KNOW...well, you paid for it!
Watch Christie Johnson beat the crap out of my hands, then do it again after she tries to quit. (This is after an hour workout, so yes, she is tired!)