Dammit! I leave for lunch and all of this discussion happens.
francopoli did a very good job of answering your question, but there is one thing that I'd like to clarify.
Pluto swinging inside the orbit of Neptune would not bring it close enough to the sun to outgas significantly enough to cause the atmosphere that we are seeing today. This is the mystery of "what heated up Pluto and caused these gasses to sublimate" that francopoli was talking about in his posts. It didn't get close enough to the sun, so what heated it up? As he pointed out, it isn't tidal interactions with another large body, the usual other source of heating in our solar system.
And to expand on this mystery:
1. Pluto has an atmosphere that should have been stripped away by now by the solar wind. This is what is currently happening to it and is creating the tail. The question is, what is replenishing it?
2. Pluto, like any other comet, is full of ices which sublimate into gasses when heated. This is what causes the atmospheres and the tails on comets that venture closer to the sun. This is where that gas is coming from, the question is, what's heating the planet?
The three common, go-to sources of heat in a body in our solarsystem are
1. The sun (Pluto is too far away)
2. Tidal forces (Pluto is already tidally locked to Charon, francopoli gave a good description of this)
3. Latent heat of formation (Pluto isn't big enough for this to have stuck around. Source: Mercury's has not stuck around, and it is bigger than Pluto).
So what's causing this heat? Well first here are some other facts that I'd like you to consider:
With all comets, as the ices sublimate off, they leave the rock behind. With smaller comets, this rock is left behind in an almost "haphazard" fashion. The rock stays where it was, and it might roll a bit downhill towards the comet's center of mass. This leads them to be to porous and misshapen (like 67P).
----Leaving facts behind, entering speculation. Everything below here is an educated guess from someone who has never studied Pluto in particular, but has taken graduate level Planetary Science classes, including (recently) one in geophysics (the relevant subject here). It is very possible that I think that I know more than I do, so take everything below with a grain of salt----
My thoughts/hypothesis/speculation that might be completely wrong (feel free to NOT read this, it might be wrong)?
I think that there is something special about Pluto that we haven't seen with other large bodies that makes it take longer to lose its latent heat. And that special thing is its composition and size. The only other object similar to it in both of these respects is Triton, but Triton has tidal heating that might mask this effect.
Remember how the other comets are misshapen and porous? Well, Pluto is big enough to pull itself back into a spherical shape (hydrostatic equilibrium). Its gravity is strong enough to pull the rocks back down and close up these porous "gaps" left by the escaping volatiles.
Perhaps some event (possibly an impact) imparted a lot of heat into Pluto in the somewhat recent past and caused a lot of ices to sublimate all at once, and now the atmosphere is slowly returning to equilibrium. As Pluto reforms itself into a spherical shape, it would create all sorts of active geology like mountains and canyons and other things that we are seeing there that we weren't expecting. This active geology would also cover up the scars of this event, much like what happens on the Earth, or possibly by the refreezing of these gasses, creating an ice layer that would hide the craters.
This isn't too unreasonable. While the Earth has plate tectonics and thus is only geologically active along faults and at hot spot volcanoes, this would be happening to Pluto everywhere, and the surface rock would be being reformed everywhere, because all of Pluto would have been outgassing and all of Pluto would now be compressing.
(CAUTION: EXTREME SPECULATION IN THIS PARAGRAPH) Perhaps the "heart" is the impact site, and the heat from impact melted a lot of the ices and created a muddy crater that had a low enough viscosity to flow into becoming flat again (no crater), and even flow some mud into the surrounding areas. I would suspect that the original impact crater was MUCH MUCH smaller than the heart, and the rest of the heart would be caused by this mud flowing out. We already know that this region resembles frozen mud cracks on the Earth, and is rich in Carbon Monoxide. This would fit with liquid Carbon Monoxide mixing with the rocks and dirt on the surface and creating a mud that then refroze. In fact, this feature is so consistent with a freezing mud flow, that I would personally be surprised if it wasn't caused by some sort of local heating, causing what I just described.
The hypothesis that I just described fills in a lot of the gaps in knowledge and questions that we currently have about Pluto, BUT keep in mind that I am by no means a Pluto expert. There are definitely things about Pluto that people have studied and that the experts know about that I do not, and it is also very possible that one of those things could throw my whole hypothesis out of the window. I'm mostly just typing this out so that I have a record of what I thought "way back when", so that I can objectively see how right I was later on, and as "food for thought" for you guys. Again, it may all be wrong, so take it with a grain of salt.