Best friends are interested in only one thing: the well being of the other. This is achingly obvious, but needs to be reiterated. Make it a point today to find something positive to say to your best pal. Make it genuine. Make it something they don't expect, like "I appreciate the way you take care of yourself", or "I really like hanging out with you", or just "Thanks for letting me be who I am". It's really simple stuff. But it brings great rewards, especially if your SO is feeling a bit of a challenge.