It's Played for Laughs, but think about this: A child trying to get the love and approval of his or her parent figure. He or she does great things, but the parent doesn't even acknowledge it. Gru's childhood probably wasn't the most happiest of times
And his father is heavily implied to be dead; and so the only parent he has is a mother that isn't exactly the most supportive, emotionally sensitive, or even motherly woman.
How about Miss Hattie's treatment of the girls in her care, which was greatly shown in her response to the "mud pie" Edith made for her:
The "mud pie" implies that Edith wanted to get some approval or even a "Thank You", but got shot down.
The expressions of the girls' when Gru gives them back to Miss Hattie. Agnes clinging to Gru's leg, begging him to not send them back. Edith is too angry to even speak and doesn't give Gru one last look before leaving. And Margo is obviously heartbroken, but still thanks Gru in a sad voice; gets even more teary eyed when she's comforting Agnes.
Judging by Margo's expression, she really was hurt that Gru was giving them back. She was really starting to think of Gru as a father figure....and then he just gives her and her little sisters back.
Agnes: Don't let her take us, Mr. Gru! Tell her you wanna keep us!
The worst part might be that, by this point, Gru didn't even want to return them (it was Dr. Nefario who called the orphanage to have them recalled, as he felt they were distracting Gru from his great Moon-stealing plan). In fact, while he was still bad, he thought he'd just ditch them somewhere once he managed to steal back the shrink ray; but since they've grown on him so much, he didn't want to part with the girls anymore. So being forced to let them go was heart-wrenching for him too.
Gru's expression after finishing the "Sleepy Kittens" story as he reads the part about the kittens' mother saying "I love you."
Gru:(Reading the last lines from the book) "Good night, kittens, close your eyes. Sleep in peace until you rise. Though while you sleep, we are apart, your mommy (stops momentarily and looks up) loves you with all her heart."
When Gru kisses the girls goodnight and Margo tells him she loves him.
The fact that Margo is the first to say it to him, despite being the most mistrustful of the whole idea, probably due to her being the eldest.
The Minions, Stuart and Jerry crying after the girls leave with Miss Hattie. You can just tell that they grew very close to the girls.
Gru's complete depression over the girls leaving. From his lonely lonely expression at the dinner table to his sleepless night in the bed. Then, you come to the sudden realization that he knew it was his own fault over what he was feeling: That he gave up probably the greatest thing in his life, something that he subconsciously always wanted: A family.
Gru having to break the news about the bank's rejection to the Minions and Dr. Nefario, though it does suffer from Mood Whiplash almost immediately afterwards.
Gru's flashback to when he was eight years old and watching the moonlanding on TV:
Gru:(Excitedly) Mom! Someday, I'm going to go to the moon!
Marlena:(Not looking up from her sewing) I'm afraid your too late, Son. NASA isn't sending the monkeys anymore.
(A dejected Gru looks down sadly)
Margo is afraid of crossing to Gru's ship after Edith and Agnes have jumped.
Gru: Margo, I will catch you, and I will never let you go again!