what causes you to care?

What causes a person to care so much about another? What happens to make you willing to walk to the end of the earth and back for another person? And to accept that heartbreak is inevitable, but to keep on trooping through? Trust is such a huge factor in a relationship, but it's also supposed to be mutual. Along with being honest, to care, love, want the best for them even if it means you get hurt in the process, etc... and what makes us want to share everything with them and for-go what we would usually do in the past and do the most random things that are so unlike us that causes that person and you to be so happy? Why are each others feelings always intertwined? If one is happy, sad, frustrated, stressed, hyper.. why do we jump into that pool of feelings with them just to make sure that they aren't alone? And why is it that when just a simple argument happens or something goes out of line, why does it hurt so much more than it should? And why is it that when we know it would be so much easier to just go ahead and get out of it all, why do we keep pushing through and keep hope? What is the cause behind all this? Simple. You're in love.