Beware of flatterers and flattery. Usually there is an ulterior motive in such cases. Some proverbs and quotes regarding this are:

Many know how to flatter; few understand how to give praise. - Greek proverb.

Beware of one who flatters unduly; he will also censure unjustly. - Arab proverb.

When a person who never flatters you suddenly discovers all of your qualities, he either wants to con you or he needs something from you. - Spanish proverb.