Once they find each other in Pandaria Sig is SO happy to have Leon back, but at the same time he’s terrified of doing ANYTHING that might bother Leon in the slightest. He’s been given a second chance and he’s desperate not to fuck it up. Not formally together at this point, things become more and more awkward in part because he’s trying too hard. Leon knows Sig’s uncomfortable but aside from subtle assurances doesn’t try anything. He’s still not 100% sure this is the best idea but he’s willing to see what happens.
The details of their first kiss are blurry but it was likely a spur of the moment thing. They finally defeated some scary big thing in Krasarang probably and in the excitement Sig gives Leon a kiss without thinking. As soon as he realizes what he’s done he’s hit with waves of dread, but Leon takes this candid opportunity to reciprocate.
It’s not the first time they’ve kissed but it’s the first time it meant something to both of them.
That settles it. Krasarang is for Lovers.