Subtext is amazing, it's a very naturalistic/realistic thing that is very easy to recognize, but for me, difficult to consciously (and tastefully) create.
Subtext is our tell, the true face behind our mask. Subtext is like beams of light from our id flickering out through the cracks in the boarded up windows of our Super Ego. Our true feelings, our darkest desires, etc.
It's like when you ask a good friend if he is over his ex and he replies
"I was never really that into her"
or he volunteers before you even ask "I'm so over my girlfriend!"
Subtext. His words betray him. Betray his truth.
The first response shows that he is really trying to sooth his pain by downplaying the importance of her to him...which would might infer means he was very much into her. The second response shows that she's already on his mind and he has to fight his own feelings of regret or melancholy with a public rebuttal (against his inner monologue) of anger and denial.
There are much more subtle and playful ways to create subtext, not just in dialogue, but with imagery, and action, but, this is a good example of everyday subtext.
Sometimes a light is just a light, and this is not here.