Why does Protestantism not really believe that good theology helps people love better and corrupt theology corrupts loving better?
If I remember correctly, I'm pretty sure the NT epistles are filled with encouragements and scourges for the church to think better so it can love better.
In the first place, I don't know that one can make such a blanket statement about "theology" and its place in "Protestantism". Certainly I cannot imagine a Reformed Christian make such a claim. In fact, I'm not sure I would, either, although for different reasons. In my own case, I would say that "good theology" and "bad theology" are not so much causes of better or worse loving; rather, they are correlate to them because they are effects of a similar cause - sin.
To say that "good theology helps people love better" is to give to theology far more power than it has. Certainly, theology is an important part of the life of any Christian. Reflecting on what it is we believe and why has been something we Christians have done since the beginning (if you don't believe me, check out the New Testament; there are some writings there that are good examples of this kind of thing). Yet, theology is nothing more than reflection. We are looking in a mirror and seeing what is behind us. If a Christian is honest enough, one could say that even the most profound theological statement is as corrupt as the most confused (kind of like Barth's insistence that all dogmatics is prolegomenna). Like any reflection, it is not quite what one sees, being reversed and limited (one needs multiple mirrors to see, for example, one's back).
It is grace that offers us the opportunity to love better, sin that keeps us from so doing and even loving worse than we might otherwise. Theological reflection is not a substitute for love or grace; it is only a fully human (therefore sinful and grace-filled simultaneously) act.
His second question includes a Bible quote (a bonus!):
AND that we can steer toward perfectability for perfectly good reasons and to good effect?
2 Peter 1:3,4:
"His divine power has given us everything we need for life and godliness through our knowledge of him who called us by his own glory and goodness. Through these he has given us his very great and precious promises, so that through them you may participate in the divine nature and escape the corruption in the world caused by evil desires."
What say you, Methodist?
I'm not sure what this latter has to do with the former claim that theological reflection can be a means of grace. Certainly, John Wesley would accept the principle invoked here in 2 Peter, but only as indicative of the three-fold grace of God, moving us toward perfection in love. Please note that this is not a "perfection" in the sense of being a good little boy or girl all the time. Rather, it is, through a process of disciplined living, opening oneself so fully to the grace of God in Jesus Christ that one acts out of love - love for God and love for neighbor - at all times. Wesley insisted that such was indeed possible, and we United Methodists affirm this even today. Yet, such "perfection" has little or nothing to do with theological reflection. It has to do, instead, with a disciplined life. Theological reflection can be, and perhaps even should be, a part of that process. In the end, however, theology is an effect of grace, not a means for it.
