I am seeing in some apps, like Swarm and google play and many webs, flat-design based, that companies are using hexagons to contain images, but I am wondering why is this happenning? and whay it feel more comfortable and look better than using circles or squares.
As for the UX aspect of this question, I'm going to have to say I very much doubt their effectiveness. If anything, perhaps it's because they cut off a slightly smaller part of the image within the same square dimensions:
Having said that, I'm pretty sure Swarm's reason for using hexagons relates to their branding, which draws from nature's very own bee-hive hexagons.
.edit: As for why they "look better" - well, that's taste. And there's no arguing about that one! One could argue they look different though, since not everyone uses them. As such, it could cause them to stand out more, because we pick up on patterns that are broken!
Swarm has a solid branding reason, as shown above, and there are surely a couple who use it for a futuristic feeling, but for most sites, it boils down to novelty. It may feel more exciting, like when sites started using circles rather than squares to contain faces. (To be fair, there's a iffy but real case for circles as more humane.) But there's nothing special about hexagons. They're a shape. You can do some slightly different things with them. But that doesn't mean they actually look better.