Why does my ex want to be friends?

My ex broke up with me last spring. I finally got over it, then around Christmas he asks if we could talk. He apologized to me for the breakup and some of the issues he caused leading up it, and asked if we could, at the least, be friends. I agreed and now we a friends and spending time together with other mutual friends. It feels a little weird though. I still feel tension and some anger, and he has been having severe anxiety attacks since we began speaking. He doesn't think I am the root cause of his anxiety, and he still wants to be friends and battle through it, even though it would just be easier to part ways. I feel bad for him, because I've gone through what he is feeling and it sucks. I just don't understand what his motives are for being friends and that makes me somewhat anxious. I care about him a lot, but I've made it clear to him that I do not want to get back together. We did talk it over and he says he just wants to be friends and he is open to the idea of us dating again should feelings begin to spark in the future, basically if it happens it happens and we go from there.
Why does my ex want to be friends?
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