Threesome mmf: Why does she respond to him differently?

I convince my girlfriend to have a threesome with my best friend. At first she was reluctant but ever since she had sex with him I noticed some changes. Like She used to never like giving head but now she always seems eager to suck his dick before we start or while I'm doing her. But every time he does her she always stops sucking me off like she's too distracted for long periods. He's not bigger than me and I made her cum plenty of times, but lately she's always responding to him more than to me.

I asked her who's better and she said we're both good, just different. When I asked why she seems to be more into it with him she said that's just my paranoia talking. She makes it seem like it's no biggie when we all have sex but I can tell she looks forward to it more than she's letting on.