Comma Rule: Do Not Join Independent Clauses with a Comma

Independent clauses are grammatically complete sentences. To join two of them, use a semicolon.

Correct
Grandma and I went to the park to watch squirrels harass small dogs; grandma hates those small dogs.
I, Acornicus Floofy-Ear Squirrel, will defend the kingdom of The Tree from the threat of the dog-menace; I will give my life for The Tree.

Independent clauses function as a sentence on their own; you join independent clauses when they are closely related, for emphasis, or when one clause summarizes the other. 

The unwavering grimace of the legendary fluffy-eared war-squirrel; it prepares for the unmitigated carnage of battle and probably some crackers tossed by children.

Incorrect
I am Dave, I like sports.
I am Sports, I dislike Dave

These examples don’t work because the two independent clauses are joined by a comma which makes it a comma splice. Avoid comma splices.