As mentioned by rh75 it's not like Arata instantly had his harem. He worked for each and every one of them. Which means he's also going to do everything in his power to make sure they are happy, even if that means making everyone his enemy. I personally think that harem mc's that are completely oblivious to everything that's going on around them is far worse. The girls tend to get hurt far more with them than mc's like Arata and Issei from DxD who actively seek a harem. In fact in the case of DxD all the members of Issei's harem have been mentally and physically saved by him, a prime example being Akeno. On the other hand I can't tell you how many mc's faces I've wanted to smash in because they're so freaking dense that they hurt and or break people with every word they say.
Also it's not like Arata goes completely against the dream "Kindness, occasional coolness, idiotic straightforwardness" minus the occasional he's all of that. He doesn't start out the best either, true he has obvious potential to become that but at the very beginning he looses the person closest to him because of that exact same potential that he has no control over. So throughout the majority of the series so far he's constantly trying learn how to use it so he can get her back. Not to mention now that he has a fairly good idea of how to use it he has officially started on the path to a Magic King. Which means he also has to find a way to defy his fate of becoming the destroyer of worlds.
So it's not like he's a perfect character, in fact he has a lot of flaws. I think what makes people like him so much is watching how he overcomes them while still pursuing his goals in style. Instead of the "Oh I have so much going on I'm only going to focus on one person's feelings and completely ignore everyone else's" It's much more pleasant watching someone say "So what if I have a crap load of things going on! I'm going to get everything I want and make sure that it ends with everyone happy no matter what it takes!". So maybe more than the "dream" it's the "ideal" that everyone wants to be and do that draws us to Arata.
Sincerely, this User has Described what I think it is, without doubt, the main reason why arata is a MC that is easy to get drawn to him........ Personally I don´t like perverted MC a lot, but, what I hate more it´s a MC that isn´t honest with himself, if he wants to do something( and that something its a not a crime) why the hell they must be doubting for half of the series to take a choice that everyone knew they had and would do, (also, in several cases, they make the choice just by a few words from a secundary chracther). I think that, in world like this, when you do your best not only when trying to do something for you but for someone else, you´re a great person, because those kind of persons are endangered species (Again,in this world).
P.S: This is, obviusly mi opinion, I dont intend to offend someone with this comment, actually this is my first commment, so if I did said something that was rude to anyone apologize in advance.