There is no must of smiling during a prayer.
On the contrary we are asked to concentrate and focus...
The Muslim should be solemn and submissive during his prayer and he should keep away from everything that may prevent him from being solemn and submissive, because focus is the heart of prayer.
as Allah wants us to be humbly submissive during prayer (23:2).
So according to this fatwa smiling in a prayer doesn't invalidate a prayer, but it's not recommended if it is a kind of disturbance from the main goal of praying.
On the other hand we are encouraged to face our brothers and sisters and in general our fellow men with a smiling face (if we can), according to ahadith like here in sunan at-Tirmidhi (which you also may find in al-adab al-mufrad of Imam al-Bukhari).
And Allah knows best.