Some people only do things for others when they know there is something in it for themselves. This is a selfish outlook on life and I can guarantee that you will make few genuine friends like that.
My grandmother was one of the most selfless people I have ever known. She would help anybody she could without ever asking for anything in return.
When it came to later life when she was in her 90s and totally unable to look after herself, she reaped the rewards ten fold. She would get constant visits from her children, grandchildren, great grandchildren and so on. She was never lonely like a lot of elderly people are.
My gran had no care for materialistic things in this world. In her spare time she would go out collecting door to door for charities. Her happiness came from helping others and making other people happy. She was a true saint of this world.
At her funeral the local church had to put up marquees outside the church to accommodate the hundreds of people attending.
So as this lovely quote suggests, do good for others and you will be rewarded in ways you never expected.