For me it was a combination of things:

1. Being out with my husband and kids at the park one day and hearing him remark that he thought he saw somebody he knew from work (whom I hadn't met yet), suddenly I experienced this strong fear of having to meet his coworker because I was 5 sizes overweight and didn't want the guy to go back to the office the next day and tell everybody about my husband's fat wife.

2. Realizing that I wasn't able to shop at "regular" stores anymore, and even XL wouldn't fit any longer.

3. A casual acquaintance who originally met me a few years ago when I was 3 sizes smaller, upon seeing me again for the first time in a few months (during which I had gained at least one full size): "Oh hi, I'm Glen...and you are?" He was absolutely sincere -- he really didn't recognize me! I was mortified but played along like I was meeting him for the first time to save us both some embarrassment.

4. Getting my husband to finally see his doctor and be tested for sleep apnea (years of horrible snoring, stopped breathing many times in one night) only to have him tell me one month later that my snoring has gotten just as bad and that he has recently heard me stop breathing in the middle of the night!

That last was a real kick in the pants.

Of course there were years of being asked by complete strangers when I was "due", of course that mostly happened when I wasn't pregnant!