Friendly, by the way, can be the exact opposite of friendship. Gillian Sandstrom has shown in a variety of very clever experiments that if you strike up a friendly conversation with an acquaintanceyou're certainly unlikely to have a substantial relationship with them. Still warm interactions can be really meaningful and that we should do it more often.
How do friendships change as we get older? Should you join a bowling league? And is Angela more important to Mike than Mike is to Angela?