Why do people tend to be shy to praise, when they are only too happy to complain?

This can have detrimental consequences. Take, for example the world of open source. People often get discouraged because it seems to them they only get the complaints, never the praise [http://www.geocrawler.com/lists/3/SourceForge/11114/125/7038861/].

Sticking with this example, perhaps we feel reticent about wasting a developer's time, and it's only when we're motivated by a bug that we take a bold step. Note that such motivation could be selfish ("I hope my program will be fixed!") or selfless ("Being alerted of this will help the developer improve the software for everybody"); but judging by how many duplicate bugs get reported on BugZilla, it may be the former in many cases.

Is this a case of seeing only the ObviousEffects of our actions? A bug report helps improve the program. A complimentary note has no clear effect. But it can have less tangible benefits in making people feel happier.

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Needless to say, this behavior is not limited to open source and online communities.  Such is human nature.

: By all means, please add non-geeky examples to make this page more accessible to all! :)

To give genuine praise at each suitable opportunity is a skill, and one not easily obtained.  

It is a paradox of musical performance that pianists and organists at weddings rarely get praise, while praise is common for pianists and organists at funerals.  

See also RewardReputation, UnsungHero, AttentionSeeker