Can't figure him out- riding advice PLEASE

So I'm pretty inexperienced with sex. I've have had multiple partners but I didn't stay with any of them that long so we only had sex maybe 2-5 times. BUT NOW I've been dating a guy for a lot longer than the others and I've gotten to know his sex style and he's gotten to know mine which has obviously resulted in better sex for me. But there's one thing I can't figure out; if I'm riding him from underneath(missionary), he goes wild and I can tell he really enjoys it but then when I think I've got a good rhythm going, he'll start his own thing and it totally throws me. Do you think he wants to be more in control or does he just feel like switching it up or maybe it actually isn't that great? Help!