It was gravity that brought them together. They hadn't wanted to, but they had been thrown together by a STRANGE twist of fate, whether good or bad they had no idea, but it was wonderful to them anyway,
That first meeting had started it, the third Hokage having seen an opportunity in pairing up his two most secluded shinobi. Then having passed the objective to Tsunade with his death.
After the chunnin exams, Iruka found himself in Kakashi's path at every opportunity, whether in the mission room, or meeting up with Naruto for ramen. He was always there.
Kakashi however, was just as confused that he kept running into the chunnin sensei as Iruka was about running into the jounin.
Their relationship didn't start till after Naruto, Sakura, and Sasuke moved forward, leaving awkward holes in the two men's hearts. They talked it over and eventually found themselves in the others presence that helped to stem the loneliness that the both felt in the teens' absences.
Eventually, they grew accustomed to seeing each other every day and went out of their ways to get together on a daily basis. They were oblivious that their actions were those of a couple courting each other, but the rest of Konoha wasn't as ignorant to it as they were. There was a betting pool on who would make the first move. Kakashi was the favored one gaining the most bets, but Iruka was the one to kiss Kakashi first and confess his feelings.
The gravity that they felt for each other was just too strong to ignore for the both of them, and even though some tried to separate them, they simply got pushed out of orbit, whether by giving up or forced, which was the favored tactic by Kakashi.
So they had each other, and neither was lonely anymore, gravity may have brought them down, but it also brought them together.
They couldn't complain when they had their ups and downs, because they owed everything that they had and treasured to that natural occurrence.