Relationships (of any form, platonic, familial, romantic, etc.) are like parking meters. I understand that it's an uncommon comparison to make, but bear with me for a minute.
Parking meters run on a fundamental concept: time and the lack thereof. You use a parking meter when you want to put yourself in a situation, whether you're going somewhere for business, pleasure, obligation, or choice. You go in knowing that there's a time limit, an expiration date for when your activities are going to come to an end. This seems to be a concept that's easy to grasp; for a while, you have time to do whatever you want, but eventually you won't.
But the biggest part, undoubtedly, is that there's choice involved. When the time starts to run out, when you know that inevitably things will come to an end, you have two choices: to let the time expire or to feed the meter. You can scrape around for more quarters and dimes, knowing that you will have to continue to add more coins and delay the inescapable conclusion to your activities, or you can accept that everything comes to an end and leave the spot open for someone else to fill.
Sometimes, it's beneficial to have added time. You have more time to spend living how you choose, with the power of choice under your belt. You can choose to do whatever you want with your borrowed time, but that's exactly it: it's not yours to keep. It's all rented time, time that you eventually have to understand is necessary to return and to keep only in your memories as a fondness you once had.
Not everyone has a place in your life, and sometimes you have to give them up even when you have extra time to spare. Just because someone is around, just because you care for them or maybe even love them, that doesn't mean that they deserve the extra quarters and dimes you're willing to add. If you're lucky enough to have people in your life that are worth the extra cents, by all means give the effort they need and deserve, but if they aren't, let go.
Letting go isn't simple or easy. We all want things that make sense in life, that make us happy and make us feel like everything is falling into place, but that's precisely the thing: letting go doesn't bring any of that. It's messy, so incredibly messy, and not easy, and has an infinite amount of bits and pieces with it that complicates the situation. But ultimately, letting go is necessary. Letting go shows you what is meant to be in your life, and, more importantly, what isn't.
Relationships are complicated, as are the people in them, and sometimes you need to move on. Your car isn't meant to be parked in the same spot for too long, and neither are you. Sometimes, you've just got to save your cents, move along, and spend your time on other people and places.