Other people have said it, she wants to get to know you and see if you're worthy of her time and affection and serious in your intentions, and ready to make an effort.. Basically, she's testing you.
Many guys (and some girls) can fall in love very quickly, and some can fall out of love quickly too.. So by pacing it she's giving you a chance to get to know each other better and see if you're compatible. Basically, she's protecting her heart.
She might like you very much and be shy around you. Or, she might not like you that much. (Or at all.) These can be sometimes difficult to tell apart, but you can see if her smile is genuine (she may be interested in being friends or something more then) or if she rolls her eyes when she sees you, or something like that..
There were also guys I wasn't interested in that I wasn't playing hard to get, I was simply avoiding them! lol There's a difference!! (I could laugh and joke with them too, if they started conversation, in fact much more easily than with someone I was really attracted to... So just rapport in conversation doesn't necessarily mean anything. Also, if she tries to set you up with her friend, she's probably not interested.)
By 'playing hard to get' some guys can be more interested because they see you have standards and such, and a life, and they don't take you for granted. Some shy guys can unfortunately be scared away, though. Not all girls are well-versed in flirting, and even those that are, sometimes just use it to 'test' their powers over men.. or may get scared if they start feeling for you too much, or it doesn't seem to be a good time or something.. But if you really like her you will pursue her.. (Maybe we read too many Cinderella stories??)
lol Sounds complicated, no? I wonder how some people still manage to date!! lol!!