Parents often make their children say 'please' and 'thank you' and send thank-you cards. In effect, they make their children apply compliments mechanically. Certain politenesses are appropriate in certain situations, period. The merit of the people involved is irrelevant.
The same thing can be observed, say, on sports teams where players are told to cheer on their companions, and chastised if they do not, even if they didn't feel like it or considered the event unworthy.
Some people realise this mechanical approach is silly, and then reject compliments and saying nice things altogether. It's difficult to accuse such people of wrongdoing. They aren't hurting anyone. All they are doing is failing to take action to, possibly, help others in a somewhat minor way.
However, even if there is no burden on people to say nice things, they still should do it. It must be merit-based and applied when felt, to have meaning. But fanmail (even very short ala "nice post"), comments on blogs that say "keep up the good work" (hint hint), or telling a friend "I'm having fun," when deserved and true, is valuable. It is encouraging, and we should like to make our friends feel good.
One might not see why this is particularly important. However, one reason it comes up is that I am generally against, say, telling one's friend "I like you" (see previous post). So, in the absence of normal things like "you're my friend" and whatnot, it is especially important to be active in expressing genuine, useful information like "I'm glad we did X today" or "that thing you said was brilliant" or "you look beautiful today".