They say that you need to be 'good' in this birth, the current cycle of life and death you're going through. When you ask why, the most common (and miffed) response is that 'so you reap the benefits in your next life'. Huh? Come again?
To counter that, I'd put forth my view: First, let's assume this whole reincarnation theory to be true. I'm not gonna be around then (the way I am now), to know or understand that when 'X' happens to me today (this is the next birth talking), it's all due to my deeds / actions / thoughts that were part of my life in the previous birth! How, then, does it matter how I am today?
Agreed, it counts if you believe that you're gonna reap the rewards / make things miserable for yourself, right here, right now - or okay, maybe ten years hence, but in this very life cycle! How can my karma today dictate my so-called next life?