It's always the burden. I lost my best friend like this, twice. Everything was just perfect, I was happy to have a friend like that to whom I could tell everything, I felt great with the guy. Then once he told me that he was in love with me, and he wanted us to date. I didn't want to date him, not because I didn't like him, but simply I liked him as a friend, there wasn't any sparkle, any physical desire whatsoever for me. I explained to him that I really liked him as a friend but had no intention of dating him, and that I valued real friendship much more than dating. For a while we carried on being friends, but things were awkward, and then once he got drunk and sent me a message that he couldn't go anymore like that and because he knew I would never make up my mind and date him, he couldn't talk to me anymore, because he couldn't bear it, stuff like that. Since then, I only once received one drunk message from him saying pretty much the same, but we never talked to each other again.
Almost the same thing happened to me again a bit later, with my friend from high school. We got really close during a school trip to England, and things were a little bit romantic with us, but we never started dating. Then he started dating another girl, and I stepped back to give him space. He broke up with that girl and came back to me, and we became friends again. He was in some difficult situation with his parents divorcing quite nastily, and he suffered from depression and so on. We were spending a lot of time on ICQ talking about pretty much everything, sometimes he would get into the mood that he threatened with suicide if I didn't come to his house at night and so on, but I was always pretty rough with him when he did this, and he stopped immediately. Or when I went away for a day and left my phone at home, I would find 29 missed calls from him when I got back, but we were still friends, just going for walks and talking, we had the same hobbies, and music taste, and opinions. Then once he told me he wanted to invite me for a date. I explained to him the same thing as to the previous guy. He turned it into a joke and pretended he didn't ask, but eventually in a month or so he started to date a girl and our friendship ended again. The girl was my opposite, always concerned about him when he threatened with suicide, and in a year he basically drained her and broke up with her, after which she was devastated and he started to drink and do stupid things. Then we went on another school trip for a week, and we got close again, talking rather normally to each other by day when he was sober, but the nights when everyone was drinking he usually got drunk and then doing some propositions to me like insisting that I would sleep in their cottage with him, or I would come to my cottage and find him in my bed etc. By the end of the week it got calmer and we were back to our "normal", but there was still the drama to it as the girl he had broken up with was our classmate too and we would have her breaking down whenever she saw us two together. After the trip he eventually came back to her, but this "coming back" periods lasted until the end of high school when we went to different unis, and only saw each other a few times, but I would say we still sort of keep in touch. Recently I met some friends from high school and we exchanged news about the others, and they were surprised when I mentioned him and told me that actually I was the only person of the whole class he was still talking to. While the previous guy really upset me, this crazy roller-coaster actually didn't affect me, probably because I was the dominant person in that relationship and I was always in control of it, even though I remember actually feeling jealous of that girl when he was going out with her (not because I wanted to date him, but I didn't like her for stealing a friend from me). His best friend once told me (after we had a few drinks) that I "sexually frustrated" the guy, that really made me laugh
So yes, with me it's always decided immediately if I want to be just friends with the person or if I want to be something more. I don't grow from one to another. And it really upsets me if the other person feels the other way and tells me after such a long time, I always feel like they were supposed to tell me at the very beginning
I understand that maybe they weren't sure at that time, but still all I put into the friendship goes to waste afterwards...