Why are women so difficult?

my GF and i ended things today over a very trivial matter. my friend offered to pay my motorcycle off for me. i told him i would give him the bike. he said no its mine and its his favor to me for convincing him to not kill himself. i told my girlfriend this a couple months ago. she didn't like it and felt its against her principles to accept money to pay for something that i own that is used for fun. we ended things today and she told me that this decision i made with my friend was the biggest reason why! why are women so difficult??? its a petty reason. there are a 1000 other reasons out there and this one ended my relationship after 8 months? she is very firm on this too. i suspected other things but it came back to just this 1 reason. its very petty and i am starting to believe relationships are just a major waste.