Whoa, pretty much everything I can say has been said here so maybe I'll be a bit concise.
What made me fall in love with the character is her kindness, her selfless hard work to help and improve, her strong, passionate will to keep on going even though she faces such tough, what in my case would be morale breaking obstacles on her path.
You can also add most other incentives that have already been posted, and most certainly been dug up right from peoples hearts.

Before I noticed Lisa, my morals were, well, I wasn't a good person if I look back at it.
I felt so emotionless, dull, empty before I fell for her ways.
Now she makes me feel like I can take anything, no matter how tough it is, real happiness and what I see as the "love".
I quote from Gustavo "the love is in whom helps you to be the best one to be human…".
I reacted strongly to that, because it's true... she's helping me in being the best man I can be. Asking myself "What would Lisa do?" always helps, and even if I just need strength when I'm under pressure, feeling down, anything, thinking about her cheers me up, giving me the strength to keep going.
Her childishness, ability to be unsure and sometimes get mad, but still being able to see and correct her wrongs, to be shy and modest (and being all around cute

) is what perfects this treasure. I don't know how she could be any more perfect, she is what the true definition of "The perfect human being" should be.
Alas, she is not real, but her spirit lives within me nevertheless, and that's enough to fuel me. The real thing though, would be heaven.
And now I wrap that up! *LOL*, it wasn't quite as concise as I thought it would be. These forums are great anyway, they've allowed me to read about people who feel/felt the same way as me(I had no idea, made me think I was completely insane or maybe vice versa), and allowed me to get this heavy load off my chest and settle down, so I feel much more at ease now

As you probably imagine (and know) I don't get a chance to talk about these things with anyone else.
I'll probably continue skimming through (means reading carefully ^-^) these forums for a long time. Hope to see lots of interesting topics and fanfiction. Half way through the Wings story, and I love that kind of stuff

Might just write a few of those myself when I have spare time.
Cheers!

/ Kasper