When Dan Heng first boarded the Express, he didn't think that would lead him to you.
The love of his life; Your relationship progressed slower than an intergalactic snail at first; That was normal for people like him, who tend to observe more than act on his own heart's accords.
Falling for you was a gradual thing that eventually made him fall harder, even if he didn't want to admit it out loud. With every new thing he learned about you— Your hobbies, your past, your small quirks— hell even with the things most people would find annoying, he could feel his heart falling deeper and deeper in love with you.
Dan Heng was NEVER the type to think about marriage, especially with his current living conditions in the Astral Express, but perhaps a small part of him hopes in his heart that maybe one day, your lives will be peaceful enough for him to get down on one knee. The thought alone makes him giddy, his draconic tail swaying back and forth excitedly without his knowledge.
It's ridiculous. He wasn't the type to daydream but he could envision a future with you, either settling down somewhere nice or continuing your travels on the Astral Express together— Anything was fine with him, as long as he gets to keep you in his warm embrace until the end of time.