I follow most professional sports, but they don't even begin to compare to college sports, in my opinion. I could list a thousand reasons why, but I'll try to keep it short.
Game day atmosphere. There's nothing like 100,000 screaming fans at a college football game with chanting, fight songs, marching bands, etc. College fans are without a doubt more passionate than pro fans. Most of the NBA games I've been to are dead silent for 99% of the game. It only gets loud if there's one of those races on the scoreboard for a free donut or if the scoreboard says "Make Some Noise!!!" Then it'll quiet down again after that. The NBA is the only sport I know of that actually has music playing during the game. NFL games are a lot better than the NBA, but it still only gets loud during 3rd downs.
Tradition. A lot of college teams have been around for over 100 years and have had the same uniforms and fight songs the entire time.
Gameplay. College football and college basketball teams have a wide variety of offenses and defenses, while the NFL and NBA teams are all very similar and cookie cutter.
Player and fan loyalty. As a fan, you're always going to cheer for the same college team no matter what. It's not uncommon for pro fans to move to another city and start cheering for new teams. Players will always be loyal to their colleges. I see former Ohio State players wearing OSU gear and attending games all the time. College players choose who they play for. Pros are either drafted or play wherever they make the most money. Not to mention they'll play for several teams over the course of their career, while most college players only play for one team.
Every game matters in college football. College teams can't afford to lose a single game. NFL teams can sometimes lose half their games and still qualify for the post-season.
Like Steve said, a lot of pros are playing for money and won't play hard 100% of the time.
I see the frequently changing rosters in the college game as a good thing. It's awesome having a class of new players come in every year while having a chance to honor the departing seniors. I enjoy seeing the players develop over their 3 or 4 year career in college. And it's not like pro teams don't have constant roster changes. The difference is that the departing players from your team are now your enemies and when your team gets a player that you previously hated, now you love him.