OPINIONSTEASING: I personally love watching/reading about relationships that are bit flirty. They have that type of relationship that makes Sango want to slap him in the face, yet kiss him all over. He is a pain in the butt, but she loves him sort of thing. And it's not in an abusive way at all, just in a fun-teasing type of way. I really think that this adds character to their relationship instead of just having everything go smoothly all of the time. Their little road bumps happen from Miroku poking fun at Sango's insecurities, or just being a dumby.
PROTECTING: The two of them both are always willing to protect each other. Sango chases after Miroku as he gets himself into multiple dangerous situations with demons who dress up as beautiful women. She knows she can get injured, but she goes in anyways to save his dumb-butt! Miroku is also willing to jump in at any moment to save her. One of my favorite episodes is when Sango is facing a water-demon and Miroku tells the demon to stay away from [his] woman! Sango blushed and was happy, and I blushed and was happy.
GROWTH: Both of them change a lot through out the series. This is because you grow with the people you choose to spend your time with. They both had different backgrounds and stories to share, and because of that, they both grew in different ways. I think they both learned what it means to be a great friend and a great companion as well. They learned together to be a mental-lover and supporter before they added anything physical in, and thats another reason why they grew so well together.
SACRIFICE: With love comes some forms of sacrifice for the other. Sango allowed her Hirakotsu to be completely destroyed in order to save the one she loved. Miroku was willing to be sucked in by his Wind Tunnel many times as well in order to save Sango. In the end, they were both willing to die together if it meant they could spend eternity together in the after-life. This is just showing their dedication to each other in a bit of an extreme way, but still realistic for the feudal times.