Why did she lead me on, then proceed to reject me?

Not my story, picked it off a confession board. But I'm just interested in everyone's opinion in it. Why did the girl below lead the guy on, then proceed to friendzone him?

--- Story follows:
What happened is that there's this girl I've known since freshman year. I've liked her since then but did not make a move for various reasons, one of which is that she was attached. We however do hang out quite often but at this point it was still in a group setting. As the years go on and her relationship broke down (through no fault of mine), we began to get closer. We start going out together. Study dates, movie dates, dinner dates, the works. However to anyone and everyone who asked, we were definitely "not dating". I however begin to like her for real and was starting to hope for something more to develop. After gathering the courage, I finally managed to get myself to ask her about where exactly do we stand.

I was told no.

Granted she was extremely nice about it and took a pretty roundabout way to say it but at the end of the day, it was still a no. I was told about how she wasn't ready, how we've been friends for so long and how I should find someone better. I took it there and then and shrug it off best as I could. I mean, it's normal to reject and get rejected by people. For a week, I felt i was slowly getting over her and settling into my place in her friendzone. Within that same week though, I found out that she's out with someone else and they may be getting pretty serious.

It felt like a fucking punch to the gut. Why the heck did she give me that entire story about not being ready when a week was all she needed to get with someone else?
Why did she lead me on, then proceed to reject me?
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