Why do I like the idea of a relationship but a real life chance of it makes me uncomfortable?

I am just trying to know if it's normal among others. To explain what I mean, I like the idea of being in love. However, if someone approaches me real life I don't feel comfortable. A guy told me yesterday that he liked me and although there were no obvious reasons to turn him down, I declined it politely. That just seemed the right thing to do. He tried to reason with me and I didn't like that. I have a coworker who I have to meet daily and since past few days I've been seeing him acting differently with me: getting all flustered and eyes all sparkly while talking to me, staring at me first while entering a room, coming up to me whenever he sees me and many more things. He also asked me once if we could go out for coffee or so after work (because he said he didn't like going alone) and I had said that had lots of work that day. I fear it getting anywhere because a rejection will make things difficult for both of us as we will be at this place for at least coming 5 years. I had an online relationship once and meeting him in person felt scary.

Is it common among people?

Why do I like the idea of a relationship but a real life chance of it makes me uncomfortable?
Post Opinion