Ashwathama to Duryodhana's - Kinds of Friends

Discerning persons say that there are four kinds of friends, viz.,

1. Those that are naturally so,
2. Those that are made so by conciliation,
3. Those that become so through wealth, and lastly those
4. Brought under subjection by the exercise of power.

http://www.sacred-texts.com/hin/m08/m08088.htm