Main reason is because I hate it but I know you can get over that.
Another reason is my arms. They will never be normal. I lost over 100 pounds twice and without surgery (which involves incisions from the armpit to the elbow) I will never be comfortable in a tank top, sleeveless dress, or bathing suit. I might wear them but I will probably be drinking so that I don't think about it.
I will never want a picture taken that is not just my face. Never. Because there is no way to hide them. Even with long sleeves. I could be a size 2 and I would still look like a whale because of my arms. So what is the point?
I know my husband my friends etc don't care. You think I'm pretty and nice whatever. But about 75% of the world discards me as a useless piece of flesh because I have fat arms. Pretending that doesn't bother me is really hard sometimes. And working out is just a nice long spell where I get to feel helpless, disgusting, and worthless.