Tapas purifies our system and strengthens our system. Purity and Strength - that is the purpose of tapas. But tapas can also bring a big ego in a person. Unfortunately, people think, by doing tapas they will become very great. If someone has to fast for long periods of time, he must have had lots of impurities. Out of proportion glorification of tapas leads to ego.
That is why the next thing to be done immediately after tapas is swaadhyaaya. Swaadhyaaya is self-study. Why do you fast? Is it just to show-off? Is it in competition with somebody?
There is a story. There were two neighbours who were meditating and doing some tapas. God came to one of them and asked him what he wanted. The man said that he wanted whatever He would give to his neighbour. He asked God if He was visiting his neighbour too. God replied that he certainly would, considering that both of them had started tapas on the same day. God pointed out that He had come there first because he had started his tapas an hour earlier. He again asked the man what he wanted. The man asked Him if He could avoid going to his neighbour. God refused, saying that it would be an injustice if He did so. He had to go there. God, once again, asked the man what he wanted. The man replied that he wanted twice as much of whatever God gave the other man.
Then God went to the other man and asked him what he wanted. He asked i