The entire story itself is heartwarming. Enough said.
A few quotes from Howard:
"Thank you for being the very first one in this place as far as I could recall, to be kind to me and to even care in the first place. Thank you for having the time to listen to me." (Chapter 10)
"You do realize that we're both old already, right?" he said, insisting me to divulge the deepest, most painful details of my bitter life. "It's important to share what you know before you forget them yourself so that others may learn from you. That's why I've been very open to you even though I've only known you today. Of all the sea creatures in the world, you're the only one who's ever been kind to me. You proved to me that I'm not going to suffer alone forever in that bleak alley. I really mean it.". (Chapter 11)
Some quotes from Plankton too:
For the first time in so many years, I've felt sorry for someone in a big city that seemed to be filled with apathy. (Chapter 9)
"Back in the place I used to live in, no one would ever describe me as someone kind and caring," I replied, brushing off an undeserved compliment. "To tell you the truth, I've lived almost all my life as a bitter failure, rejected by society. I lived almost all my life alone. No one was ever kind to me, so I was never really kind to them either. It's hard to be kind to those who hate you and make fun of you." (to Howard, Chapter 10)
I was happy to meet Howard, though, because I have seen for myself that I'm not really alone, that in all these years, there is actually someone out there who actually shares my problems. (narration, Chapter 10)
For the first time in my entire life, I knew that I was doing something right for the right purpose. I knew now that my new-found purpose is to make a difference in Howard's life, to reassure him time and time again that everything will be okay. It was the least I could do after all those years that I myself had been repetitively reassured by a machine who loved me unconditionally.(Chapter 10)
"The important thing for me now is to be able to apologize to him one last time before his last breath. Whether he forgives me or not is no longer important if he is already a dying crab. The best gift I can give to an old friend I've been in a war with for almost my whole life is an apology in his hospital bed. Besides, I made the decision to go on this journey back home, and there is no more turning back." (Chapter 13)
"But I chose you," I replied. "I know how much you hate that alley. It's nothing but a dead end to you. I hate to see you like this, you know? I want you to be happy one last time before you move on and forget me, or everyone else. Now, let's go on, pick a random pipe, and hope that it's the way to Bikini Bottom. It's the only thing we could do, and it's better than not trying. Karen once told me that a life lived for oneself is a pointless life. It was my mistake to live alone and live for only myself. Now that we have each other in this city of apathy, whatever you have done is no longer pointless." (Chapter 15)
This line from Mr. Krabs:
"Thank you, Sheldon... And I forgive you as well. In fact, I should be the one apologizing for making almost your entire life miserable. When I finally get out of this hospital, would you like us to re-establish Plabs Burger? Of course, we'll follow MY recipe."