Whether for sex, money or other favours, people tend to take advantage of others when they can. It can feel very saddening to realize that someone you trust does not have pure intention towards you and are only relating with you for what they can get.
Here are signs you are being used:
Having a relationship with someone (be it romantic, friendly, work) does not automatically give them the right to make endless demands. It shows that the only time you are useful to them is when they need whatever it is you provide. Keep an eye out for that person in your life who cannot seem to stop asking things of you.