Why programming is fun?

"A favorite programming moment is when I get to fix a bug in code that already has a good unit test. Such work is often akin to putting golf balls into a neutron star's gravity well; I get this can't-go-wrong feeling."— Kristofer Skaug. 

1975, Fred Brooks wrote The Mythical Man-Month: Essays on Software Engineering; he listed the five reasons (“Joys of the Craft" chapter) why programming is fun: 

The sheer joy of making things. 

The pleasure of making things that are useful to other people. 

The fascination of fashioning complex puzzle-like objects of interlocking moving parts. 

The joy of always learning. 

The delight of working in a tractable medium. 

Now after 30 years the programming has even more fun. With invent of PC and Internet the excitement has grown further and there are some more points that can be added to original list. (At Brooks’ time PCs and Internet did not exist.)

Teamwork:- The enjoyment of working with other programmers in a team. 

Hardware:- Being able to play with leading-edge computer hardware. 

Debugging:- Helping to debug complex software systems that someone else built. 

Internet:-Using the Internet to collaborate with other developers on projects and problems. 

Getting paid to have fun. 

Being able to program anywhere, anytime, with my choice of platform, programming language, and architecture. 

Some more items for the list from other sources: 

Because it's a combination of intelligent and creative work. 

Being some kind of nerdy superhero. 

Nearly instant gratification. 

The pride of seeing my work used by other people. 

The thing about it that really hooks me is taking a machine that was designed for no purpose in particular and making it do anything I want. 

The benefit that it brings to users in making their lives easier. 

One comment and one quote deserve special mention:

"A favorite programming moment is when I get to fix a bug in code that already has a good unit test. Such work is often akin to putting golf balls into a neutron star's gravity well; I get this can't-go-wrong feeling."—from Kristofer Skaug. 

"If it isn't fun and profitable, what the hell are you doing in the business?" by Robert Townsend, Up the Organization