Reasons to lose 100+ lbs

Your cholesterol will go down.
Your blood pressure will go down.
You will have cut your chances for diabetes and/or breast cancer in half.
You won't ache when you get out of bed.
You will be able to focus on other things (rather than obsessing on how fat you feel)
You will fit nicely in movie seats, theater seats, ball game seats and even restaurant booths.
You will enjoy trying on clothes. (Even tee-shirts look better on you)
You can do more things that are fun (Like I go down the water slide with my kids now... Never would have before)
You will have learned to deal with your feelings... to allow yourself to feel.
You will sleep more soundly (not sure why, but it's true)
You can do more things in one day since you won't get winded just going up and down the stairs.
You will feel excited to start a new sport or hobby (I'm training for a 5K and a Diva Dash)
People who don't know you will be more respectful of you... like clerks, and cashiers, etc...
You will have empathy for others unlike someone who's never been there.
You will no longer be embarrassed for having to stand on the scale, like at the Dr's office.
You will be an inspirations to those who think they can't do it.
You will learn to trust yourself and that feels great.
You will gain strength and be able to participate in life instead of just watch it happen.
You can go swimming in a swimsuit without thinking people are staring at you.
You can order at a restaurant without thinking people are judging your order.
You will feel more feminine when you throw out the elastic waistbands and wear a belt.
You will know you have done what most can't and know you can do anything you put your mind to.
You will no longer feel like you are ruled by food.
You'll get excited to see people you haven't seen in a long time, rather than avoiding them.

Oh dear... I could go on... and on...

Wow, Julie has a good list. I would also add that you should do it because YOU want to.
I personally have to place the 'big' number on the back number and focus on smaller goals that I can achieve in a regular amount of time. You won't have to lose all 100+ pounds to start seeing some of the rewards of weight loss