žalostiti

1. to sadden; to upset
   Njegova ravnodušnost me mnogo žalosti. — His indifference really saddens me.
2. (žalostiti se) to be sad; to grieve
   Nemoj da se žalostiš zbog takve sitnice. — Don't be sad over such a small thing.

The cause is usually introduced with zbog + genitive: žalostiti se zbog nečega.