Why...

The best people who play MS are the older people, you have no reason to be ashamed. No one needs to know you play either.

Well, if you ask me, "manly" games like Call of Duty (shooters actually) doesn't look so good either, because "REAL MEN!" are not playing computer games at all, they are shooting real stuff at the army, or fixing cars or whatever...
People who play games are usually the creative type, and as such, you will rather play something colorful and creative, and not a mash of guns and blood.
I don't have statistics, but i'm just guessing most of the Call of Duty players are 14 year olds that think it is "cool" to shoot stuff.

I have no shame and tell everyone in the world I play MS. I'm 24 years old, and think this game has some major advantages other games would never have:
1. It's really colorful! No "manly" guns and blood and realistic look... It just looks good.
2. It's fun and addictive, and these two go well together.
3. It's ENDLESS! You can play as much as you want, and start whenever you feel like it all over again, with a new class.
4. You design your own character to look how you want it to be.
5. It's easy to play.
6. And most importantly - it's SO easy to play, you can actually do a lot of stuff while playing!
I listen to audio books and lectures, watch movies and TV shows and talk on the phone while playing, heck, I could work while playing!
This saves a lot of time, and gives me the opportunity to do these things, since most of the times i'm getting bored after 10 seconds while doing only one of them.
I can't listen to an audio book without doing anything at the same time, so I MS at the same time, and it works great!

So if my friends ask me why I play it, I say these two:
1. It's a hobby, and you should invest time and money in your hobbies, since they make you a better and more interesting person.
2. I do many stuff at the same time that I wouldn't be able to do alone, like listening to audio books and lectures.

It's a great game, I just hope it will develop with a better Story, AI, Quests and EXP curve for levels 100-200, since when you reach levels 100 you don't have anything left to do, and after playing an Explorer, Dual Blade, Aran, Evan and Resistance to level 100, you have nothing left to do anymore.
And that's a little sad.

In conclusion, it's a good game, and that's more than enough a reason to play it.
If your friends like Call of Duty or fixing cars or whatever, be happy for them. You like this, and they should respect that.
If they can't respect you liking something and investing time and money in it, I dunno how much "friends" they are.