Because clinging to positive things leads to more greed, more mental instability and discomfort as whatever your cling to will one day disapear or change, making you uneasy.
This is one of the 3 marks of existence : anicca, which means inability to maintain things to your satisfaction.
This will ultimately lead to constant rebirth, immoral acts, and rebirth in the lower realms, as you might be tempted to do immoral things to satisfy your cravings (if not in this life then in the next).

Your question is funny as it seems you are actually far from the buddhist mindset, and you only want less of what you dislike and more of what you like, which is the common mindset of all living beings in samsara.