Losing weight is tough. We all know that. It shouldn't be, but it is.
And when losing weight, there's nothing better than having someone support rather than sabotage your efforts. And the thing is, if you're mainly friends with 'thin friends' (you know, the ones who can eat whatever they want and still stay in the same jean size) they can often not get this difficulty.
Unless they're a really rubbish friend, then they're going to want to support you and often don't know how to.