Flaring is good for developing finnage, and keeping the rays strong afterwards. However, a Long Finned betta can blow their finnage if they flare too much. The caudal can split between the rays. There are some males that tend to chew their own finnage when they flare for too long of a time. If the reflection is so clear they want to flare, you can put a background on the tank. You might try changing the light, or putting a reflector of some kind on it to change the angle of the light, or to dim it a bit. Having a male with enough energy to want to flare a lot is a good thing. It means he is healthy. I'd suggest you fix things so he flares when you want him too, and not all the time.