I loved them from the Pilot and it is not that there was a scene, where anyone of them explained explicitly the way they feel about the other one.
It was pretty hard for me to not notice their chemistry - it is really rear to see such thing on TV nowadays.
I mean at first I started to watch these series, because it is sci-fi and I am huge fan of this genre.
After the pilot I was more fascinated by the relationships between the characters, than the alien stories.
Liz and Max were somehow perfect couple, without anyone being near to perfect.
They had their flaws, they had their mistakes and doubts and second thoughts. After all they were teenager and yet most of the mistakes they did were because they are trying to think more about the other, than about themselves.
For example at first Max did not want to be with Liz, because he wanted to protect her. He was not right, because it was her choice, but he did try to make it for her.
Then she decided to let him follow his destiny without even realizing that it is actually his choice and if he loves her, he will want to be with her.
And then this whole cheating, that was not cheating for me

. I mean they were not together at the time, so no one cheated technically, but still they both got hurt, because of their bad choices.
And despite everything Liz was the one that was able to connect with Max in New York and warn him.
He saved her life and she saved his life.
Their love did beat everything and even after everything they did, they still loved each other, they still helped each other and still were behind each other most of the time(well except the end of Season 2 and the investigation of the Alex murder. This is also an example of their indestructible connection - Liz was right and wrong at the same time. She did risked their alien status and their secret, but she was also right, Alex was murdered by aliens and because of her determination to find out the truth, they all realized the truth about Tess. Max was right about protecting his identity and his family, but he was also blind and not willing to see the truth. He preferred to put his head in the sand, instead seeing that maybe there is something strange about Alex' death).
So, what I am trying to say, although maybe I got lost in the process

, was that there is no need to be flawless relationship, with no problems, no fighting and no differences between the partners. A good relationship is the one that makes people to go through everything and besides that to love each other.
It is what makes people to put themselves behind the loved one, it is what makes people to be there for each other, to be willing to help each other and to be able to trust each other.
And to me, Max and Liz had all.