I totally agree with you!
Even in the manga it surprised me how much time the author gave to Rintaro's mom and later to the whole family as well. It just makes you care for all of them equally as much.
This series is one of the reasons I started opening up to people and actually trusting them with some of my deeper emotions and thoughts, which is why it's also my favorite manga and anime (once it's finished).
The characters feel grounded, down to earth and just super realistic. They talk to each other like any real human would without feeling unnatural.
Please let this series never end! (or give it a proper ending like it deserves!)