I have been friends with a girl at work for more than a year. I have always seen her as a friend and I am pretty sure I never did anything to lead her on. I am going to switch jobs in a couple of months. I told her this a month ago. From that time she has been acting a little strange. She always comes by my desk during work hours, constantly texts and calls me. But I thought she was reacting because I was leaving from the workplace.
A week ago, her sister got married. I was at the wedding and it was amazing. Myself and a few friends at work had the best time! She was really happy. The next day she told me that she had feelings for me. She also revealed that she had a crush on me for more than seven months before we started speaking. She clearly says that she is in love with me and wants to spend the rest of her life with me.
She is a really sweet girl, but I never considered her more than a friend. I would like to tell her "No", without ruining our friendship. She really supports me and helps me in making decisions. Also, her sister has just moved out of the house so she seems a little sad. Yesterday, she brought up the topic again and asked me what my answer was. I wanted to tell her "No" but just don't want to hurt her feelings.
How do I explain this to her without her getting hurt/our friendship getting ruined?
EDIT: She also expects a reason why I am rejecting her (if my answer is NO which it is)?!
UPDATE: Everything went well. I moved to another country after a few months. I told her that she's a great person and I hope she understands that I have other things to do in life and I am not ready for a relationship. Also I told her that we're friends now and will always be. I told her that there is no guarantee that if we get into a relationship, that it will go on forever. But this will go on forever. Its best if things stay the way they are. I also said, I have a great deal of respect for her and she'll always be a special person in my life. She was kind and gracious enough to understand. It's been eight months and we're still great friends. Thanks for all the advice!