Polish, Czech, Slav... similarities?

Good evening.

Purely as a hobby, i've decided that i'd like to pick up a European language (I already speak a couple Asian languages).
I initially wanted to learn Czech, but i was comparing populations that speak languages in Europe and found that Polish outnumbers it and that it also has a lot in common with slavic languages.

tl;dr:  Anyone who speaks Polish or Czech, or any combination of those Slav languages, are there similarities?  'ppreciate your comments.

Having such similar languages, why do you have to kill each other?