Just turn the question around. Ask him what's so good about something that he happens to dig. Most of the reasons he'll give you'll just be able to go, "So?" to and demonstrate the absurdity of asking what's so good about someone's hobbies. A hobby is almost always by defnition pretty pointless.
I like Ts becasue they don't take up much room, make the best examples of spiders that I can keep in captivity, are beautiful, and I find them fascinating to watch and study in their cages.
If someone doesn't like tarantulas, all of those reasons mean exactly nothing even though they mean everything to me.
You could just give him the old "If I have to explain it, you just wouldn't get it." line, or simply, "It can't really be explained, you just have to experience it."
My favorite is "They are awesome predators! What else can I say?"
tell your friend the great thing about tarantulas is that you worship them as your gods complete with sacrafices and they will grant you the powers to call down the wrath of the tarantula gods on all the boneheads of the world an reap the revenge of spiders everywhere killing all the people and leaving their valuable property for their choosen human deciples(sp?) ... if hes still there at that point then take your two pointer fingers and place them on either side of your mouth making a hissing noise while jumping up and down. If he still is there after that then feed a cricket to one of your T's in front of him - that ought to do it.
I am fascinated by their growth. I love to see how much they grow with each molt. I love the way their patterns become more aparent from s'ling to adult. And how some patterns change from s'ling to adult (as in c. cyanopubecens and many avics).
I am fascinated with their web building.
Watching an avic. go nuts for a moth is great.
I love watching them walk.
I love it when they climb up my neck and lick me behind the ear. (You should have seen the face of the person that I told that today!) =D