It seems with all the evil acts he has done in his entire adult life.
From murdering and restoring Bor only to have him killed.
Or having Asgard be brought to it knees by Osborn and his cronies.
Or during Defalco run where Loki killed or was it seriously injured a friend of Thor and numerous other examples.
Simply put Thor put no thought into it. He has a rose colored memory of when the two were kids and Loki being occasionally nice. Given everything Loki had done and all the times he claimed to have reformed Thor should not believe him. Since Asgard was still a pile of rubble from SIEGE and Loki was the key instigator Thor was an idiot to think the other Asgardians would just accept him. He was even dumber to not realize why everyone was mad at Loki. Thor also spit on the graves of everyone Loki killed during SIEGE by bringing him back. Loki gets off scott free and a second chance while everyone else remains dead. Even wiping his memory chances nothing because Thor should have some idea of why Loki turned evil and the first place and should realize by keeping him in an environment where he is hated will only have the same effect in the long run.
I don't see kid Loki as currently vindicating Thor. Its more like a lucky shot that just happens to turn out right so far. Loki was not born evil so for him to have his memory wiped would revert him to a more benevolent state. However, if Loki does turn evil again a lot of the blame will fall on Thor both for Loki turning evil and the damage Loki causes.
I don't see Thor bringing Loki back as some grand act of nobility. Loving Loki still is fine, but bringing him back in an environment where he is almost bound to turn evil again along with the insult it is to the rest of Asgard does not speak of nobility to me.