I'm certain our piggies trust us and know they never have anything bad to expect from us - there are those dark moments of baths or claw-cutting, but they won't sulk for very long after that.
Recently, I had a bad day and was very unwell, I think it might have been pain in my knee. I was quite depressed and my bf got our guinea-pig Patch out. She insisted on climbing on my lap and snuggled up in my arms. I think she might have sensed that I wasn't ok. Sparky used to do that as well.
Our couple of little Spotty and big Basil might not love each other, but they are certainly attached to one another. I think the most horrible thing we could do to them is separation and if we take them out of the cage, we always take them out together.
Our couple Cotton and You in the kitchen definitely need a lot of love, especially Cotton. She will cuddle up on our laps and then she seems to be in heaven. She is the lightest and I think she needs a lot of attention and appreciates it very much.
I don't think they feel love, though. That's something we did in a Philosophy course once. Love means you put the other person's well-being before your own, you want them to be well and happy and do whatever you can to make them well and happy. However, I think animals are generally too selfish for that. They might take care of each other, but I don't think the guinea-pigs can consciously try to give us love.
But then, so don't little babies and those who have kids did love them before the babies were old enough to give something back, right?