Moving on

Why is it so taboo to say you miss someone no longer in your life, or a moment gone too quickly? I guess I'm feeling nostalgic; two of the people that I've felt the closest to in the past how ever many years... well, I feel life urging me to let them go and move on. I will miss them, and though their influence on my life will not be nearly the same, I hope it will still be there in small remnants of what I was used to. They were the ones who've loved me, and told me so; and because of that (and life experiences), I clung to them like a person dying of thirst to a glass of water. They cared about me in ways that I needed (although not always necessarily ways that I wanted).
I feel sorry in one aspect; that I will never truly be able to express the depth of my gratitude or what they mean to me. I believe that it is time for me to move on, and because I believe that, there is peace. I feel ready... which is leaps and bounds away from just a year ago, when I frequently panicked at the idea of them not being in my life. I know now that this is normal and healthy. Leaving people in the seasons they were meant for, instead of dragging them along behind... it's life. But there is some sadness. Enough so that I can justify saying a heartfelt "I will miss you" when the time comes.