Because in heaven, love will have nothing contending against it to interfere with it being full on 24/7. Here in this world, prior to "Kingdom Come", we have the sin nature in the flesh, the "law of sin and death" working IN the flesh and through its members, fussing against and trying to interfere with what the Holy Spirit (and Love) seeks to do in us. And the reality of the matter bes that sometimes the flesh wins out, because that bes the reality of being born a sinner in a sinful world. This reality bes meant to prepare us for heaven -- not remove us from the guaranteed place there which Christ's life, death and resurrection make freely available to us.
WHEN we walk in the Spirit we do not fulfill the lusts of the flesh -- so it stands to reason that when we do not walk in the Spirit, we most certainly WILL fulfill the lusts of the flesh (sin aims, proclivities, programming, desires, etc.), because had we been capable of not doing so in our own strength we would never have needed a Saviour in the first place. Most Christians bes fightsy the wrong battle. They think the battle bes about fighting sin. Nothing could be further from the truth -- Jesus already fought and won that battle decisively FOR us. We may CLAIM it as our own because of Him and because He won it. No, our battle bes, instead, learning to walk in the Spirit instead of in our natural (carnal) selves. Because "keeping sin out" -- and here it must reiterate this should never be our focus in the first place or else we bes already fallen from grace -- does not happen by dint of programming the brain a certain way or deciding in a hermetically sealed fantasy chamber that we will, before we actually do so. Nope. It bes a matter purely of where temptation finds us: walking in the Spirit or walking in our flesh. One way wins, the other loses, and all without us lifting a finger. Nifty huh? P.S. anyone on the wrong side thinking this to be a matter they must press out themselves, well that bes why He bes patient and invents forgiveness.
sorry to end abrupt -- MUST sleep, it suddenly crept up and smashed on its head.