Riyad Us-Saliheen (Gardens of the Righteous)

Chapter 178
Desirability of Entering the Mosque near one's Home upon returning from a Journey for offering two Rak'ah Prayer

988. Ka`b bin Malik (May Allah be pleased with him) reported: Whenever the Messenger of Allah (PBUH) returned from a journey, he would proceed straight to the mosque and perform two Rak`ah of (optional) prayer.
[Al-Bukhari and Muslim].

Commentary:  The performance of two Rak`ah of optional prayer is an expression of gratitude to Allah for the safe return. Naturally, one feels happy on such an occasion.