I just read the ending of A Human Friend, and it was so tragic and bittersweet at the same time. But i guess we understand why you did it, when it comes to war their relationship was already at risk, especially with the warp traitor knowing. And it also made TC realize that caring its not a weakness but what defies a friend from a fake and it gave him a purpose that isn't as senseless as the war, I'm not sure what is that purpose he made to Lana's grave but maybe is to protect the home of his friend and find a more fulfilling life and maybe , since Lana wouldn't want him to be sad and alone, he will find the 'passenger' he yearns for and even if they can't replace Lana they will allow him to heal, like Buster the dog TC got in a comic.
And maybe he also made the Autobots question whether ALL deceptions are bad or a lost cause, and there is more to them than meets the eye. For us as well as TC was the first deception that i read about that wasn't a complete bad guy.