Upadana Briefly

 In general you want to let go of all the things you cling to, because that will cause them to be around more, rather than less. 

 If you try to cling to a waterfall, you get wet, then you dry and the water is gone. More water will keep coming off the mountain whether you cling to the water or not. Each moment is impermanent but the nature of the moments are permanent.

 If you try to cling to an annual, you interfere with germination, and don't get seeds you can plant next year. If you try to resuscitate the extremely dead flower, you interfere with getting out there and planting new seeds.

 If you try to cling to signs and significations rather than the thing itself, you blind yourself to the thing itself, creating a delusion of impermanence. If you try to cling to especially impermanent things, you stop them from being renewed.

 It's not a moral thing, it's merely ineffective. It's not some deep spiritual thing, it's merely a neuronal bug that happens to be especially common. It's the nature of things to have power law distributions, not be evenly spread. As such, one or two of the bugs were going to be especially common. This is one that so happened to be. 

 You cling to things because you're afraid they'll leave. What if you let go and they stayed anyway? Would make you feel kinda dumb, right? That's probably why you don't let go - you already know the grasping is unnecessary.