What do you think causes the crackle and clicks we hear?

Dirty records. 

No mention yet of groove wear? I am shocked and disappointed.

Actually, groove wear does not cause pops or ticks. It slows groove velocity. Dust ground into the surface does that along with static and overload. This is a problem we have all learned to live with. It just goes with the territory. If you don't like it go digital. 
MC you really need to go digital. This stuff is getting to your head or whatever you call it:)

I think pops are easier to understand but clicks and crackle noises, what causes this?

Some of course are caused by damage to the LP surface and static electricity. But not all. Many ticks and pops are caused by radio frequency overload of the phono section. The RFI is caused by the operation of the cartridge, which has inductance, interacting with the capacitance of the tonearm interconnect cable.


Many phono sections simply are not designed to deal with the RFI generated in this manner! So they overload and you get a lot of ticks and pops. When I say 'many' I mean most.