The reason I enjoy giving fast offerings is that it is the easiest way for me to "impart of your substance to the poor, every man according to that which he hath, such as feeding the hungry, clothing the naked, visiting the sick and administering to their relief, both spiritually and temporally, according to their wants." Mosiah 4:26.
When I pay tithing I include a fast offering (most of the time). I'm at peace with the amount I include for fast offerings, however, there is no legitimate reason for me not to give more.