People who do good in the world usually end up with a positive view of themselves. Helping somebody out can have a big effect on your feelings of self worth.
Of course, this does not mean denying your own wishes and becoming a person who is taken for granted. Putting yourself last in a negative way is what many people with low self esteem have been doing for years. When you choose to help, be clear that it is your choice. That way you always stay in control of what you do.