Because most individuals do not like facing their own feelings, nor the ones of others.
Nothing keeps anyone from being truthful, yet kind.
"Hey, I am sorry, I have offered you a lift at the beginning and I didn't mind doing so now and then, but I am starting to feel resentful to drive you home each time when I live in the opposite direction.
I don't mean to make you feel bad, but I thought it'd be better to be honest with you so we can keep appreciating each other, rather than for me to grow resentful, I hope you understand."
A reasonable person would understand, apologise or maybe explain why they were looking forward to be driven home (could be she has some chronic condition and this lift meant a lot for her to be in less pain/be less tired) which could have made them look as ungrateful and such.
An unreasonable person would try to convince you to still do it, hold a grudge against you etc...
No reason to feel bad for denying the unreasonable person those lifts and unfriend them altogether.
But having been honest would grant you in exchange the truth on who they are, while granting them the truth about who you are.
Isn't it nicer than having people pretending while grinding their teeth from their dislike of their own fakeness?