Assuming if one of Loki's biggest motivations in helping Thor is to gain revenge for Frigga having been supposedly murdered by Malekith, my question would probably be as to why Loki would still care about Frigga considering on how he's essentially abandoned and let go of those that were in his life while he was still an asgardian prince.
Frigga has never abandoned Loki. In the prequel comic, it was revealed that she made a psychic connection with him before he came to Earth. Also in the prequel, she greets and hugs him upon his return to Asgard. Then Odin tells him that it is only because of Frigga that he is not being executed as punishment for his crimes. It's also been mentioned in some of the articles/interviews about TDW that it's Frigga who supplies Loki with books, a bed, and other comforts in his cell. She loves him, sees the good in him, and thus it's fully believable that he loves her in return. She is his lifeline, and if
she is murdered by Malekith or Kurse, which I think is a given at this point, with all the revelations/spoilers/hints given/leaked over the past months