Sanji had always liked how Usopp, despite being somewhat of a coward, always jumped into the fray when he was needed. Usopp was always there for everyone, whether it was to mother them, to entertain them, or just to keep everyone's spirits up. Sanji had always loved how Usopp put others before himself –most of the time.
Sanji had never even considered a romantic interest until Robin made a vague reference to them having separate house roles. Usopp was the mother, Sanji the father, and although at the time both boys had made various gagging motions and refused to look the other in the eyes for the rest of the day, deep down they both felt that maybe Robin was right.
There was never a doubt in Sanji's mind that he would be the first to make a move. He always just assumed that that was how it would happen, that when he finally met the girl of his dreams; he'd be the first to act on his emotions. Deep down, Sanji believed in the true romance of life, not the lust. He would have never guessed that the girl of his dreams wouldn't be a girl at all, and that they had been under his nose all along.
Usopp had always believed that he'd be with someone like Kaya for the rest of his life. That he would have someone who listened to all of his stories and get lost in them, they would hang on his every word. But at the same time, he felt like someone like Kaya would be too much of a sister. Usopp wanted someone he could protect, but at the same time made him feel like he was needed. He always just assumed it'd be a girl, because what guy wants to be protected?
No one believed that Sanji and Usopp had anything in common, so everyone on the Going Merry was shocked when word got out about the two of them. Usopp was mortified for about three seconds, before realizing that these were his friends, they really didn't care if he was in love with Sanji or not.
Sanji had denied it for a minute, trying desperately to convince Nami that he didn't actually love Usopp, but she had seen it, and she saw straight through his denial. Nami had no doubt that Sanji lusted –maybe loved her, but no one can control whom they fall in love with. Usopp just happened to be there at the right time.
The information of the two lingered, everyone knew that they were together, maybe even dare to call them lovers, but no one ever did. Any affection between the two was so discreet that unless seen at the exact right moment, no one would've suspected them as anything other than friends.
Sanji and Usopp didn't refer to themselves as lovers either. They didn't give what they had, it just happened. Whatever they felt fused together with the friendship they already shared, becoming the definition of love itself, or at least something like it.