A - Good question. The only time that fasting is obligatory for Catholics is during those times of the year that are set apart as special penitential days (Good Friday & Ash Wednesday). The Church law allows for one full meal "but does not prohibit taking some food in the morning and evening" also.
So, if you choose to fast outside of these two times it is not an obligation, but should be done in freedom. If you want to fast more than what the Church has set as guidelines you can do so, but I recommend you talk to a spiritual director beforehand. A simple set of rules to follow may be that you don't want to hinder your ability to perform the other things you are obligated to take care of. For instance, you don't want to weaken yourself to the point of not being able to pay attention in class if you are a student or take away your ability to concentrate at work. You also want to fulfill other obligations to your family, friends, etc.
Why do we fast? By denying ourselves something good, we remember what the highest good of all is - GOD. We also practice self-discipline and self-mastery, which we need in order to achieve holiness. Jesus fasted in the desert and calls us to as well.
"When you fast, do not look somber as the hypocrites do, for they disfigure their faces to show men they are fasting. I tell you the truth, they have received their reward in full." (Matt 6: 16)Peace to you in your desire to grow closer to God.
"and then as a widow until she was eighty-four. She never left the temple, but worshiped night and day with fasting and prayer." (Luke 2:37)
Yet when they were ill, I...humbled myself with fasting.” (Psalm 35:13)