I suppose it depends on the situation and how you interpret it as each person may see things differently, each of us being unique and all.
There are plenty people that I have left behind, almost always for good reasons, there have been others that I've drifted from because our lives were just on different paths, but people that I value and care for I don't simply discard and will make an effort to keep in touch with.
I feel that there's a lot more of my life behind me than ahead of me now, but the parts and the people that have been monumental in shaping me into who I am today will never be forgotten or taken for granted like the feathers that a bird moults seasonally.
My life has just taken on a huge change and yet, I still feel exactly the same for the people that I care for. As for material things, well I'm happy for them to go except for those that have sentimental value.
I'm with you on the more behind than in front thing and also about those who have played pivotal roles in my life; to forget or take them for granted diminishes their memories I think.
Material things, sure we need some of them, but most could be left behind, especially these days in a world where people want more things, most of which are superfluous.