Just because you treat others with a pure heart and good intentions doesn't mean people will always reciprocate.
Many times people will not.
Many times it will hurt you.
It still doesn't mean you should be like them.
Continue being good.
Continue doing the best you can.
Let those situations be the reason you treat people the way you do.
Let those situations show you why it's important to do your best for others.
Understand people behave that way because they have lost faith in other people.
Show them they are wrong.
More than ever.