Hmm.
I have a girl and a boy. I would think it was cute if my son did it, but I would also be very surprised, because he is very socially conscious. He would be embarrassed if a girl did it to him, I know.
If a little boy did that to my daughter, I might think it was cute. Or I might not. Depends upon the boy, how my daughter felt about it (she is NOT a girly girl and is pretty spirited and opinionated... and might find it lame or just plain embarrassing. But at age 6, she would have been kinda weirded out probably... she is very cautious with showing her emotions), whether it was an ongoing thing that annoyed my daughter or whether she reciprocated the feelings.
I will say, that my son had a little girl that he was very fond of and planned to marry, when he was about 3 or 4. Those are the kinds of emotions/statements that need to be addressed seriously, I think. Husband told him he'd need to talk to the little girl's dad before he could marry her. (Fellow church members). So he did. The Dad said to come back when he had a job so he could support a family.
I thought that was great.
It is fine for him to have a crush on Sara. But he needs to understand that she may not enjoy the attention, and if he cares about her, he will respect her boundaries of kind of attention she does AND doesn't like.
Enjoy his sweet open heart, but yeah.. he is hitting the age where people start expecting kids to filter and use a little discernment about appropriateness. It is uncommon for a 6 yr olds boy to say "I love you" like he did. Mallory made a good point about him getting a reputation as "that" boy. Something to consider.