He who has happiness, should hide it

Kell' onni on, se onnen kätkeköön.

Origin is a poem by Eino Leino.

Used to describe the finnish state of mind that one should not boast with what one has. This saying is also a sort of a belief. If you show off how happy you are, you will lose it.

A long joy ends in sorrow

Itku pitkästä ilosta.

Origins unknown.

Often used for example when children play too rough and end up getting hurt and start crying. Could be that it has originally ment "all good things come to end".