Ascetism is voluntary sacrifice. Anybody who is really in love does it - you love someone and you try to please him, you put that person first, it's natural. At times you listen to that person's type of music, and not your own; you put up with some of his weaknesses; you take care of him when he is in need... and all the time you know, and see, he does the same for you.  

Your Christian, Catholic way of life and your vocation is a love story.  

You try to show Christ you love him above all things, that you are grateful for all he has done for you, so you do things that someone who is not in love with Christ does not understand. You make time for him, you stick to your principles even though some of the other kids might laugh at you, you do good to others because he said whatever we do to others we do to him. 

This is all quite clear. The problem arises when we understand by asceticism those huge, external penances that some of the great saints did (fasting, scourging, etc...). These penances are only of value when done out of love. We know that the Church is like a body, and the good that each one does benefits everyone else in the body. So these saints, and there are many today who still do the same, did these penances to make up for their own sins, and to help spiritually in the conversion of those who are still far from Christ.  

With their penances they unite themselves to Christ on the Cross, and they intercede for sinners. We owe a lot of the graces we receive to those Catholics who pray and offer sacrifices for us.