Because I was a 27 year old, 5'10" at 217 at one point. I look at my family members and decided that's not the path I'd like to choose. Motivation, and I'm getting close to my goal weight. https://www.reddit.com/r/1200isplenty/
This is the truth. I wasn't able to achieve my goals until I started to do this. Measure, weigh, and you'll be shocked how many calories are in the things you might otherwise think aren't calorie heavy.