It was a gradual realization on my part--and a bit of an iceberg in hindsight!

To begin, I loved their relationship as it was in canon. They've got such a unique dynamic unlike any I've ever seen, and I remember my makor takeaway from season 1 of the anime being "I just want more of -them-."

When season 2 of the anime was airing I actually got distressed over the Separation Arc (I hadn't read the manga at the time so I was going in blind) because I really, really didn't want Mob and Reigen to ever break off their partnership, not helped that a) I think it'd make sense for the characters and their development and b) I'd seen so much fanart (in the English-speaking sphere, at least) of Reigen being endgame with Serizawa or Dimple (no shade at either of those ships, by the way) that I figured "maybe that's the way canon goes (why am I crying)." Should add this happened while I was strictly into their platonic relationship--I'm not exactly sure I'd call this a "sign" I shipped it, but it was an indicator of how deeply invested I was in their relationship!

Then somewhere while season 2 was airing, I -think- I stumbled upon a "what if Mob pursued Reigen when he was older" prompt or scenario somewhere that sparked -something- in my brain that had me transfixed. "Something" being "what if Mob got taller than Reigen" and "what if Mob got physically strong enough he could pick up Reigen"--I even wrote a bare bones scenario of it, and mind, this was -still- in my "platonic only" era.

I think the turning point for me was in season 2 episode 9 of the anime, when Mob collapses from relief in Reigen's arms--"turning point" here meaning "when I got curious enough that I poked around looking for art of them as a ship." (I'm a sucker for when a character can bring another's guard down so well they feel -safe- with them enough to pass out.) It was a little difficult to find the stuff that aligned with my tastes, but when I found it I went, "Oh shit, I really -do- ship it, don't I." (Thank you, spillingdown, tentacleboye, crowtoes, and many many other artists for helping me come to this realization <3)

And the rest is history! Sorry for the late response, I only just saw this in my inbox now and hope this answer was worth the wait. I'm sure I could go on about my history and love of MRM, but my eyes are getting tired, so I'll make this the stopping point for now.