Neji Hyuuga was a genius. His IQ was through the roof. He was so smart, that the one time Shikamaru had convinced him to play Shogi, it ended in a Stalemate. If someone had suddenly came along and asked everyone in Konoha if they thought Neji was a genius, every single person would've told them yes.
All but one, who to this day continues to insist that Neji is an idiot.
Neji was a genius, but he was a genius in denial. He spent every day either doing missions, training, or arguing with his uncle. Doing missions and training were self-explanatory, considering Neji was a Hyuuga, but it was the arguing part that had the rest of his clan confused. The topic of the arguments was always the same: marriage. Well, to be more specific, marriage to a certain Team Gai weapons mistress.
Tenten was not the girl for Neji. He had lists and lists of reasons why he couldn't marry her, as much as his uncle had tried to convince him otherwise.
There was no doubt that they had chemistry, and Tenten was the girl Neji felt most comfortable around. However, one of Neji's many lists contained five key reasons why he could not marry Tenten. He considered showing this list to his uncle, but figured it would only cause further complications, so he tended to keep his reasons to himself, instead making up excuses whenever he could to avoid asking her to marry him. Not that it would come as a surprise; practically everyone knew of Hiashi's plans by now.
First of all, Tenten didn't care about her looks. Although it had never been stated out loud by her or anyone else, Neji could clearly tell that Tenten couldn't care less about her appearance. The biggest evidence of this was right on the back of her head. Neji was forced to stare at those buns every day, wondering what her hair would look like cascading down her back. Sometimes his hands even twitched as he longed to undo the buns, and he swore that if Lee hadn't suddenly popped up next to him and distracted him in the middle of his staring, Neji would've jumped her from behind and undone the buns on one occasion. Now, being a Hyuuga required always looking your best, and if Tenten was going to leave her beautiful hair up the way it always had been, she could not be a Hyuuga.
Secondly, Tenten was much too nice. Other than Hinata, who was practically an outcast to her father, the Hyuuga clan did not contain many nice people. The Hyuugas were also not very touchy-feely, which Tenten's niceness led her to be. And Neji did not enjoy getting all hot and bothered every single time Tenten hugged him. Looks could be changed, but someone's personality would be much harder to alter, and Neji, for the most part, liked Tenten just the way she was. She just wasn't wife material.
Third on Neji's list came the fact that Tenten was way too determined for her own good. Even during their sparring matches, Tenten never seemed to give up, and Neji didn't really enjoy having to knock her unconscious every single time. But if he didn't, she would just keep on fighting until her heart gave out. And he did not need a wife that was going to go out and get herself killed. The last thing he needed was to have to constantly worry about Tenten's safety. Well, not that he didn't do that already...
Neji could never quite finish the third reason.
However, he didn't feel that he needed to, because in his opinion, his fourth reason was much more justified. Tenten would never shut up. She would rant and rant, on and on about things Neji didn't care for or about. Sometimes it was weapons (in these cases Neji would at least pay a little attention), but other times it was about perfume, makeup, lipstick... Did Tenten even wear any of those things? Neji wasn't sure. But anyway, the point was that Tenten was constantly talking, and Neji was constantly tempted to shut her up. And I'm sure we can all guess in what way he was tempted to do this...not that she didn't kiss back.
Which brings us to Neji's fifth and last point: Tenten was utterly and completely in love with him. Now, most people would slap Neji and call him a fool for using this as a reason not to marry Tenten, but Neji was a genius, and therefore, his logic simply had to surpass everyone else's. And Neji's logic said that if Tenten was dumb enough to love a stone-faced, emotionless, and overall empty shell of a person that Neji considered himself, then she must be a complete idiot. And Neji Hyuuga couldn't marry an idiot, no matter how much he returned her feelings.
What Neji failed to add to his list was his sixth and only genuine reason for why Tenten shouldn't marry him:
She was much too confident, nice, determined, talkative, lovable, and perfect to be forced to call herself a Hyuuga. And Neji simply could not let someone as perfect as Tenten marry someone as flawed as himself.
...Tenten disagrees. She says Neji is an idiot.