Being useful to people is not only spiritually uplifting it also helps you increase your network of friends and well-wishers. I don’t mean to say that you should always help people for selfish reasons; but then even if you get happiness out of helping people you are getting something out of it and consequently, you are being selfish, so you can’t just help being selfish, so this is not the point. There should be a balance though. Sometimes we become so useful to others that we don’t have time to do our own work. Sometimes there is no privacy left because people are always approaching us for help.
The best way to be useful to people, says this blog post on Lifehack.org, is not to make it feel like help at all. The people receiving your help should feel great after receiving your help, not miserable.
And being useful does not always mean doing work for others for free without ever expecting any kind of return. Even in the corporate world people who are highly useful are sought after because they are motivated and always take great interest in their work.