The following contains spoilers for Avengers #4 written by Jason Aaron, drawn by Paco Medina & Ed McGuinness, colored by Juan Viasco and Mark Morales, and lettered by Cory Petit.
The Avengers have spent literal decades with each other, and it's only natural that romantic relationships form between the various members. Some of these have turned out to be long lasting, such as Black Widow and Hawkeye, while others have ultimately wound up short and fleeting. The last iteration of the primary Avengers team didn't really have many of these relationships, but the romance between members of the Earth's Mightiest Heroes looks to be coming back.
After years (or more likely months, in Marvel Time™) of being Unworthy, Thor Odinson has reclaimed his title and gained an assortment of hammers to kickstart a new group of Avengers co-headed up by himself, Tony Stark and Steve Rogers. But even though the team is dealing with Dark Celestials and the end of the world, the God of Thunder still manages to find time to get some kissing in.
Now that the team is assembled (more or less), everyone has split off into teams to tackle the Final Host in different ways, along with saving Captain America from Loki. Thor heads to Old Asgard with Jessica Walters, aka the Hulk, to get answers about the invasion from Odin. The former Allfather informs the pair of what he and the Prehistoric Avengers did to the Celestials when they first touched down on Earth one million years ago. Though the Avengers of his time were able to defeat one Dark Celestial back then, it's different from the ones that Loki has with him today.
Ever the determined hero, Thor is convinced that the halls of Old Asgard contain something capable of bringing down the Dark Celestials, so his father leads the two to a heavily fortified door. "What's behind the door could end the world," Odin gravely intones, "or save it."
Before any more information can be divulged, Hulk just straight up rips the door off its hinges, leading the three into a snowy wonderland. Hulk and Thor race to acquire their prize: the blood of Ymir, the father of all Giants and the first Ice Giant. But just as they close in on their target, Thor starts to get hit with the deadly cold of being so close to the alpha Ice Giant. Luckily, Jen isn't all that affected, so she warms him up the only way she knows how in her Hulk-addled mind.
Though it means nothing to Hulk, it means quite a bit for Thor, both as the Odinson and the Thor title. Back when Jane Foster took over the Asgardian mantle, she and then-Captain America Sam Wilson locked lips in the aftermath of a battle, and subsequently became a couple for a brief while after her identity was disclosed. It wouldn't be surprising if Thor and Jen found themselves trying to figure things out once the whole Dark Celestial ordeal settled down -- or whatever counts as "settled down" in their line of work. Thor falling for fellow Avengers is sort of a thing he does, after all; not only was he in a relationship with Jane multiple times, he also briefly dated Moondragon when she was an Avenger. So maybe this Hulk/Thor ship will continue, if both are willing to try things out.
Even if they aren't, the kiss also shows that the Odinson men just can't help but kiss powerful women in their proximity. Back during the prehistoric days, Odin had a fling with the Phoenix up until her "death," so it seems that the two men just have a love for strong women who could destroy everything. Like father, like son.