Reason behind his title - ‘al- Reza'
A group of historians maintain that it was al - Mamun who gave him this title, for he was satisfied with him and appointed him as his successor. Imam Jawad (as) refuted this statement before a group of his companions, saying: Verily Allah, the Blessed and the Most High named him al - Reza, for he was the pleasure of Allah, of His Messenger (saw) and of the Imam's (as).
Somebody asked him, Weren't all your past forefathers (as), the pleasure of Allah, His Messenger (saw) and of the Imams (as).
Yes, replied Imam Jawad (as).
Why has only your father been named al - Reza, asked the man.
Because both his opposing enemies and obedient supporters were pleased with him, while this did not happen to any of his fathers, so only he was named al - Reza.
Reference: Life of Ali bin Musa al - Rida by Baqir Sharif al - Qarashi.
source : www.tebyan.net