In the Game of Thrones episode, "the Mountain and the Viper" Roose Bolton gives Ramsay a piece of paper, giving him the right to bear the house name, it makes him heir to House Bolton and indeed the North. Now Roose Bolton has shown no great love for his bastard, he even goes so far as saying that he regrets raping his mother,(not that that's a bad thing) so I'm wondering why he did it. Did he have a sudden change of heart and decide that he loved Ramsay and that he was no different from any other son in Westeros? Or did he only do it because Ramsay was the only eligible heir to the North?
At the time, Roose Bolton had no heirs. At all. His last true born son died young. True, at the time he had wedded Fat Walda Frey, but it would be a while until he could get her pregnant and see that son to adulthood. On the other hand, Ramsay Snow was showing himself to be an effective, if cruel, operator. He usurped the Hornwood lands, sacked Winterfell home of the Bolton's greatest enemy, and effectively contained the Greyjoy aggression. House Bolton was entering a crucial stage in its history. For the first time ever, they have fulfilled their ambition of becoming the lords paramount of the North. But the North is still not completely united under their banner. To do that, the Boltons need to have strong leadership, and part of that is ensuring a strong line of succession. Roose may not harbor great love for Ramsay, but he realizes that House Bolton needs him.