Acting the way 'you feel like' is sincerity. But sometimes one should overcome how he feels like and try to adopt a better behavior

Spontaneity is a great blessing for a man. To be able to behave naturally, sincerely and in spontaneity is a great comfort both for that person and for the people around him. This is because such a person is unaffected and genuine.  

However sometimes people think "acting the way one feels like" means "to express whatever it is that he feels like inside".  And moreover they regard this as a kind of "an indication for honesty". They think "I am not hiding anything; these are all how I feel inside".  However not everything a person feels like doing is correct. For that reason it is not right for a person to word everything he feels like as it is. Sometimes it might be necessary for one to bridle or overcome some feelings he has inside. That is because the things he says "he feels like doing" are the feelings present in his lower self and are the things his lower self provokes him to do.  

Lower self has both goodness and evil in it.  However to shape the lower self into the desired form, to defeat the evils and to enhance the good and beautiful things in it, is within the person's willpower.  

For instance, a person may feel like doing kindness to the people around him, to show love and respect, to say good words, to cheer up, to be conversable and talkative, to be extravert and self-starter, to make sacrifices and to present beauties. A person complies with such things "he feels like doing".  

However sometimes a person might also feel like doing some negative things; like to get all quiet, to sulk, to become dull, to keep away from people, to get angry, to hold grudges, to cop an attitude, to rant and rave, to dispute. And these are all evils that are present in the lower self. It is not right for a person to say "what can I do, these are the things that I feel like doing," and thus display such negative manners. And the fact that these were all the things "he feels like doing" is not a justification for the bad manners he displays.  

This is because those are evils that Allah had informed us in the Qur'an that He doesn't like, that were specifically given to the lower self and that one should strive to defeat.  Allah informed us about the requirements of good morality and the attitudes that a person should avoid. A person should never adopt a manner knowing that it is an evil thing to do, no matter how much "he feels like doing so". If he feels like getting angry, he should keep his temper immediately thinking that this is a morality Allah does not like. He should urge his lower self to act in the opposite manner towards a person he feels anger about, by placating with kind words and behaving friendly towards him. Or when his lower self gives him the feeling of getting all quiet, he should immediately think that this is not compliant to the morality of the Qur'an and use his will power to say "kind words" to the people around him to gain the approval of Allah.

In such situations acting in the favor of one's lower self, is never honesty, spontaneity, sincerity and unaffectedness. Honesty, spontaneity and unaffectedness; is to express all the behaviors Allah had legitimized and informed us in the Qur'an that He likes, in the best possible way and it is to live by this morality in the most sincere manner. Going by the evils in his lower self saying "I feel like doing these" would only be a sign showing the flaws in that person's character.  

Consequently an approach in which a person goes by "everything he feels like doing" is never reasonable. A person should go by every goodness he feels like doing and at times "he does not feel like doing these" he should definitely search for goodness, train his lower self in that respect and increase the tendency in his lower self towards goodness.  

In the evils his lower self provokes him to do; he should respond this feeling with the opposite manner in goodness, by using his fear of Allah, his conscience and willpower stemming from his faith.

This is a very important part of the test in the Muslim's life in this world. A person can attain the morality that Allah would approve in the extent he can train his lower self.  About the people who prefer goodness against the evils in their lower selves and who educate their lower selves by the morality of the Qur'an, Allah says the following in the Qur'an:  
 

And the self and what proportioned it and inspired it with depravity or taqwa, he who purifies it has succeeded, he who covers it up has failed. (Surat Ash-Shams:7-10)

You who have faith! you are only responsible for yourselves. The misguided cannot harm you as long as you are guided. All of you will return to Allah and He will inform you about what you were doing. (Surat Al-Ma'ida:105)

" I do not say my self was free from blame. The self indeed commands to evil acts – except for those my Lord has mercy on. My Lord, He is Forgiving, Merciful. " (Surah Yusuf: 53)