There's a low barrier for entry for me telling someone "I love you". It typically equates to "I care about you", "I like you", or can even go as low as "I'm interested in you". With my last relationship, I definitely felt like it was something similar to love at first sight, and I'd even joked with her with something like "I think I might be in love with you" shortly after meeting her.
As I get older and become more experienced, it does feel like "love at first sight" is more like loving the idea of someone rather than the person. You can see someone and take in the cues given by their demeanor, style, as well as many other factors and build them up in your head within minutes of meeting them. That idealized version of the person you "love" can live on in your mind for years, even after you've come to know the real them.