Scott Adams once said that there is no such a thing as a small act of kindness that every act creates a ripple with no logical end. But, some people choose not to donate their services because they believe that somehow it devalues them by "giving them away." In truth as Scott Allen said, there is nothing that demonstrates the value of your skills more than putting them to good use for a cause you believe in.
To serve is beautiful, but only if it is done with joy and a whole heart and a free mind. So if thou be industrious to procure wealth, be generous in the disposal of it. Man never is so happy as when he giveth happiness to another.(Robert Bulwer-Lytton)