Riyad-us-Saliheen 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]
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.