Why did this turn into a boring relationship movie?

Definitely not going to say you're stupid, you obviously get Carruth's intent--it just didn't work for you. But here's why it worked for me:
First of all, the fact that Kris and Jeff even find each other is something of a miracle, and the Sampler looks shocked when he sees their two representative pigs interacting so much in the field. This is something we might not expect, because individually they don't have much appeal. But then I began to wonder whether they're even fully alive, and watching their tortured selves slowly work up toward feeling something for one another becomes heartbreaking and unpredictable. Even before they started to track down the Sampler I thought the mystery was heavy and intense.
Now cross this with what Carruth has said in interviews--it's a film about building identity in the presence of another. Just as the scenes with the Thief were so affecting on a human level because of the VERY interesting way that the Thief presents himself to Kris ("I was born with a disfigurement where my head is made of the same material as the sun," etc), I found the middle section of the film so bold because Kris and Jeff are doing the same thing to each other, building identity, only they do it with much less agency and understanding of what they're doing. But then Jeff gets very self conscious and aware of the brittle way he's presenting himself to Kris, and I can't help but wonder whether that awareness must always be coupled with weakness, as the Thief never seemed to be very aware of who he truly was yet has such confidence. And the Sampler knows exactly who he is, which is why he is not capable of human connection at all.
Then I felt a sense of wonder watching their relationship unfold while being interrupted by the Sampler's interactions with the pigs, and I couldn't ever tell what parts of their personalities actually belonged to them.

In short I thought it worked because of the Sampler and the Thief. We're stuck without control ourselves watching the relationship, never sure where everything belongs, and yet it all fits together in the most visceral way, forcing us to think about every detail. Something we fail to do with even our own lives.

This was my exact reaction. I "get" what I was supposed to get, and it still made me want to kill myself. Which you can all now invite me to do.

Good question. For one thing, it sort of seemed to render all this confusing, jump cut happy presentation somewhat pointless in the end. Also, elsewhere it`s described as a "thriller"... where the hell is this suspense supposed to be coming from when the director is mostly occupied with trying to pull the rug from under the viewer at every turn as if he`s afraid that once the jumble is made sense of, it is recognised that there really isn`t much point to it all. And, yeah, what the hell is it all `about`, anyway?



"facts are stupid things" - Ronald Reagan

I agree with OP that the middle part of film is slightly dull in terms of plot (not cinematography), compared to the excellent beginning and the intriguing end.

The relationship aspect is too overstated in contrast to the rest of the film, which is a master of subtlety.

Also Jeff's character was a bit lacking. His background is mostly alluded to but not fleshed-out properly. We don't get to see how he came in contact with the parasite for instance (or was it so subtle that I missed it?).

These are relatively minor criticisms though. Luckily there is a lot more going on than just a relationship movie. Overall I think the film is remarkably good.

just stopping by in 2015 to say that i agree with you only because the main actress started wearing pants at this point

Does the fact that Jeff is completely absent from the last part of the film matter?

It seems to me Jeff's rather sudden absence makes it clear: i] the film is primarily Kris's story (and may need to be reinterpreted if we thought otherwise), and ii] the only parts of the relationship that matter here are the parts that affect Kris's response to the con.