I think the only problem with this is how do we figure out how to behave (think, feel, whatever) about things that don't have any concrete means of assessment? Why do you love your wife, for example, or how do we define respect?
Here's my answer: There are two answers, or possibly two components to the best answer I can give. I'm not happy with the "think, feel, whatever" comment, because "thinking" and "feeling" are two different things, and "whatever" sounds like you're being strongly dismissive of the differences. Nevertheless, I'll do my best to answer, although I doubt you'll be satisfied with the answers I have.
The first answer or part of one is this: There are certainly concrete means to assess the outcomes of our behavior, because our behavior has real-world consequences. Again, there's no claim of absolute knowledge involved, but we can all recognise the difference between the worst outcome to our actions, and something better than the worst. All of that carries a level of subjectivity, of course... but once a subjective standard is chosen, there are ways to measure how close you get to that standard. Even something as simple, subjective, and silly as "what is your favorite flavor of Jell-O" can be determined and measured on a scale of least to most prefered flavors. I know that I love my wife the same way... it is all internal and subjective, but there it is. If you are like me, you want to maximize personal happiness, and the happiness of people you care about, while causing as little harm to other people as possible. How do we get there?
That's the second part: we are evolved life forms, the product of a billion years of reproduction and selection that has led to the present moment. Every step of the way, evolution involves the selection of traits that lead to further reproduction. There's no mind involved in this selection: if you get killed in the course of the day, that's what I'm refering to as being "selected against." On the whole, it means that certain traits make it more likely that you will have offspring, and they will be likely to carry those traits that helped you survive. Humans are social creatures, which means that the evolutionary path involves empathy, teamwork, long-term mating, etc. The qualities that enable cooperation are the ones that help social creatures survive. Why not take advantage, and cheat as often as you can? Cheating hurts the group, and hurting the group makes survivial of the individuals in the group less likely. Stronger groups have a better chance of survival and reproduction than groups weakened by greed and selfishness. Cultural training tends to reinforce this inbred responses.
We're bred by nature to love one another, to feel connections to one another, and to behave in ways that support those feelings, which support the survival of our group. Some people are made uncomfortable by the idea that our emotions are in large part the result of genetic programming over which we have no control. I'm not too worried about it. There was a certain amount of pre-wired stuff in my brain that made it possible for my ears to hear, my eyes to see, and there's a whole autonomous nervous system that does stuff without any input from me at all. I have no reason to see emotional responses as any different. I'm born with the propensity for love, loyalty, empathy, compassion; my culture and upbringing reinforce it.
Maybe not satisfying, but its all I have... not that I have to think about it, on account of it is part of my hard-wired brain function. :)