Its not just "I will never fold a set on the flop!" - its more to do with the texture of this flop - you are only losing to a set of 7's or 6's - he could be doing this with TP OP Two Pair - even FD or OESD - so get your money in and push.

If you had say PP of 5's and just limped in and the board came 7s6s5h or 7s6s5s and you bet and he raised well thats a different story from the one above.