Why does he compare me to his exes?

I really like my boyfriend. He really likes me as well, and tells me every single day without fail. He generally makes me feel really good about myself. A few weeks ago, he asked how he compared to my exes in bed. I am not sure why he asked, because no one really likes to get compared to their exes. He has told me that I was the best he has had in bed since then. And today, he was mentioning his exes a lot, telling me how much better I am compared to them, and then proceeded to show me a picture of him and his ex together, and told me that she is the one he has had the most feelings for, but she was a bitch, unlike me, and that he likes being with me.

I was ok when he started to mention them, because they were a part of his past, however it bothered me when he continued to compare me to them, although he was trying to compliment me by doing so. And is it normal for guys to keep photos of their exes if they don't have feelings for them anymore? I don't keep any pictures of my exes, or mention them to him at all if possible.
Why does he compare me to his exes?
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