As far as I know, you use Genitive case when indicating the cause of something and that can be with either "от", "из" or "из-за".
You use "от" when the cause is a physical or psychological change
от страха, от радости
You use "из" for feelings, inciting positiveness
из вежливости, из любви
So I am having trouble understanding why the following prepositions are used in the following examples:
Он плакал от счастья.
Она сделала это из любви.
To me it seems that in the first sentence "из" should be used as "счастья" or "happiness" is a positive emotion, while in the second sentence "от" should be used, as it indicates she did something "because of", without it indicating she did something positive.