Rick Sanchez was the type of kid to pick up strays from the streets, the kind to see a cat or dog and just bring it home, he probably brought home someone's pet a couple of times too (either by accident or on purpose) because it looked cool or cute
Bro's first reaction was to open his arms for the snake because it looked cool and he would've taken it home too if the snake didn't start shooting
A few more examples to back it up would be Glootie and Harold the Garbage Goober (the crows too probably)