How to build up tolerance to low kicks?

If anyone has any advice to share...

Clench your teeth, relax your lower body, and take a deep breath

Do you guys train with shin guards?
Right before impact, you want to throw your knee out—basically driving the whole leg forward, but letting the knee lead the way in a diagonal motion.
It works sometimes, but with certain people, it’s a gamble; some strikers hit so hard that even if you block it, you're going to be feeling the pain for a while.

1. Don't let them land that low kick.
2. If you're gonna take one, at least move your leg enough so they aren't hitting you with full power...
3. Absorb the impact...

There isn't much room for deep philosophy here. It all comes down to genetics, plain and simple. Some people are born with a high pain tolerance while others just aren't. If you're just training recreationally—and I'm fairly certain that's where you stand—you should be using plenty of protective gear. Either move out of the way or learn how to block properly. At the very least, try to absorb the impact when someone lands a heavy low kick on you.