Before anything, I must point out that laughter is not haram, and the Muslim is not expected to be dour-faced and stern and rigid. Also joking around and telling funny stories is not forbidden.

Imam Ali (as) himself had a reputation for having a sense of humour.

The Prophet (saw) said, "A believer is fun and playful, and a hypocrite is grim and angry."

Imam Baqir (as) said, "Indeed Allah loves those who are playful among people without obscenity."

Laughter is something which is quite natural, and indeed Allah says in the Quran (interpretation of the meaning): "And ... it is He (Allah) Who makes (whom He wills) laugh, and makes (whom He wills) weep" {al-Najm 53:43}

The problem is excessive laughter!

1. The Prophet (saw) said "Do not laugh too much, for excessive laughter kills the heart (spiritually)."

2. "Woe to the one who talks and tells lies to make people laugh woe to him, woe to him!"

3. "Too much laughter corrupts the heart."

4. Imam Sadiq (as) said, "Much laughing deadens the heart. He has also said, "Much laughing melts one's religion just as water melts salt."

5. Imam Ali (as) said, "The earnestness of one who laughs too much deteriorates."

6. "A Man whose laughter increases is going to lose his solemnity and Grandeur. Excessive laughter destroys men's solemnity."

7. "Excessive Laughter is not suitable for a wise and placid man as well as a leader."

8. Imam Sadiq (as) said, "There are many people in this world who increased their laughter by joking too much or playing around & in Judgement day they'll cry more than others. There are people who increased their tears in this world because of fearing Allah's wrath & in Judgement day they'll be happy & laughing while going to Paradise."

9. "Avoid laughing out loud and doing things that would make others laugh this way & don't make a clown of yourself."

10. "Avoid talking about topics that'll cause excessive laughter even if it's someone else's words and you want to share it only."