Dave was never really good with communicating with people. Whenever he had a problem with someone at school, he always seemed to be able to rely on Rose to help him find a solution. Rose always seemed to be able to understand people better then Dave did, but whenever she was irritated with someone, he always seemed to have a solution to it.
John would occasionally tease them about being brother and sister, for they seemed like they would be, different in some ways, but similar in other ways. Of course, it became awkward between the two when they found out that they are actually brother and sister. They soon grew past it thought, because they learned to accept it, and not challenge it.
Of course, Karkat had to extend their relationship beyond simple good bugs that are always there for each other. He soon saw them as moirails, and when he tried to explain to Dave what it means to be a moirail, it fell apart.
Rose, at the time, was busy filling Kanaya's flushed quadrant, and they nearly spent all their time together. It grew harder and harder for Dave to talk to Rose without Kanaya with them, and the amount of time grew smaller and smaller.
Rose usually drunk at the time too, making it harder for Dave to talk to her. He still told his problems to her, and sometimes she would give all right advice, but most of the time; it was useless to talk to her.
It was even harder at the time, because Dave had human love feelings for Terezi, and seeing her with Gamzee really hurt him. He tried to understand troll emotions, getting the flushed quadrant, the pale quadrant, and the caliginous quadrant, but he didn't get the ashen quadrant that well. He tried to understand it, but the two people that he could ask were Kanaya and Karkat. Kanaya was always busy spending time with Rose, and Karkat wasn't that good with explaining quadrants. Dave didn't want to ask Karkat beforehand, because Karkat would have messed it up for Dave.
About in the third year of being on the meteor, Dave found out how badly Rose was hurting herself with drinking. He soon started spending a lot of time with Kanaya and Rose, trying to have Rose quit drinking.
The first thing he did was take the alcohol from Rose, but that didn't work. After that, Dave and Kanaya tried to slowly lower the amount that Rose is drinking each day. That worked fairly well, but they couldn't get her completely off.
It seemed useless to get Rose off of drinking, but the two kept trying and trying. Right before the meteor arrived in the new session, Dave and Kanaya were able to get rose to half a bottle a day, and the two felt accomplished. Sadly though, they were all teleported to different planets, so Dave couldn't stand next to Rose as they greet their ecto father and mother. He did come up with a plan though. When he is able to see Rose again, he will give her a great big Strider hug, and hardly part form her in till she completely stops drinking.