Five reasons? Let's see if I can stay under that number...
1: You don't have to worry about Adware/spyware/viruses.
Word. I think this is self explanatory.
2: Macs seem to last longer than Windows PCs
I know people who still have old beige boxes running as webservers in their homes. Yeah, they aren't the zippiest, but they still work. I used an iBook from 2001 from sept 3rd 2004, to April 29, 2005 and it worked BEAUTIFULLY, even though it was 3+ years old.
3: The Software is better.
Yeah, you say that there is less software available, but I don't think that is true in the sense that you're thinking. It seems like there is less because it isn't as heavily advertised. There are thousands of small software houses that make and distribute software. A lot of it is free/very cheap! When I've played with Photoshop at the Apple store, it's so much better than it ever was on windows. I actually read somewhere that CS was designed with Apple in mind. (but don't quote me on this!)
4: When you get it, it's ready to do (Nearly anything) straight outta the box!
Connecting to the internet takes 3 minutes, MAX (in all my experience.. results may vary!). It comes with iLife, allowing you to quickly and easily organize pictures, movies, photos, and music. Does Dell or HP ship with anything like it stock? (I've never seen anything like GarageBand on windows that came free with the system.) You can literally have a brand-spankin' new mac for 3 minutes, import some DV footage (connects quickly and easily through iMovie, or if it's a hand-held consumer digicam, import the clips into iPhoto), edit it in iMovie (seen by MANY to be the best consumer entry-level movie editing system) and pop it onto a DVD with a pretty sweet theme, without having to install 3rd party software that costs $$$$$.
5: The beauty!
I'll admit it. I'm a sucker for eye candy. The OS is so refined. Everything works so fluidly. There are tiny details everywhere that just elevate the computing experience.
6: Exposé!
Hit one key and have access to all open windows. Instantly. Or, activate hot corners, and never move your hand off the mouse! It's CRAZY awesome.
7: Dashboard
Yeah, Konfabulator exists (does it do windows? I don't know..) but having it built right into the OS is nice. Apple's widgets are so easy to use, and simplify life. I track gas prices, so I can find the cheapest. I can find it for under $2 a lot of the times.
8: The Apple Store.
I've had nothing but good luck here. While other people may deal with grouchy clerks, I seem to just be able to smile at them sweetly and get helped with any problem I have. (yeah... it helps to be a girl with that,

) On-the-spot help for ANY problem you're having with your Mac for FREE? That gets a "heeeellll yes!!" from me. It's not like repairs at best buy or compusa, where you give them your computer for 6 weeks, get charged $50, just to have it LOOKED at, and get charged $150 labour to install a $15 soundcard that you don't even need. All my experiences with Apple have been awesome. They never try sell me anything I don't need.
I could go on for hours.
and The cost isn't that much of a big deal any more.. a Mac Mini costs $499 for the base model... A USB keyboard and mouse (and USB hub, because.. you know) cost what... $20 total? A cheap display can be found ANYWHERE, bringing your total cost for a beautiful, stable, secure machine to about $570 (ohh yea.. cheap CRTs!).