Season 7 Episode 6 Spoilers
Daenerys didn't believe in Others like most of Westeros don't. So it was unreasonable to expect from her to go and fight the dead when she doesn't believe they exist and to abandon the war she is waging. She started to believe after seeing Others carved in obsidian caves on Dragonstone when Jon showed her.
But it is more likely that she went now because it was personal for her. What many fans anticipate and as she said herself, she "got used" to King Jon. And Ser Jorah is there also for whom she already have strong feelings, althought that love may be of a friend. So when she received the message they are in danger, it must have been emotions starting to work. Going there was not a reasonable thing to do, as you could see, but emotional one. Which emotions are stronger, towards Jon or Jorah, or both, doesn't really matter and we will see more of those in episodes to come.
So she didn't help them before because she didn't feel like there was reason to. She helped them now probably because of personal feelings. She will be focused on Night King from now on obviously, but this understanding came AFTER she helped them and saw with her own eyes what the army of dead is. Until then she couldn't comprehend what danger that really is. So she went there more because of those specific people or person that needed her help, than because of simple awareness that she must join the fight against the night.