Be sattvik. Eat sattvik food. Have sattvik thoughts. Okay, why?
I get asked this question a lot, and for a long time I didn't have a good answer.
The usual answer is that sattva is "good" and rajas and tamas are "bad." That never made sense to me. Rajas built every company I know. Half the founders I admire run on rajas. And tamas, honestly, is the reason anyone sleeps at night. So why does Krishna keep coming back to sattva?
BG 14.17 gave me the clearest answer I've found: sattvāt sañjāyate jñānam rajaso lobha eva ca, pramāda-mohau tamaso bhavato 'jñānam eva ca. From sattva, knowledge arises. From rajas, greed. From tamas, negligence and delusion. This is a practical argument. The guna running your mind at any moment decides how clearly you can see.
Under sattva, you see things as they are. Under rajas, you only see what you want. Under tamas, you stop seeing altogether. That's the whole reason sattva matters. Every decision you make depends on which guna is doing the looking.
Think about the last time you made a decision when you were restless or upset. You probably had all the facts you needed. The guna running the show just wouldn't let you use them well.
BG 14.10 adds something important: all three gunas are present in everyone, all the time. Sometimes sattva is dominant, sometimes rajas, sometimes tamas. You move between them constantly. You are never permanently one guna.
The useful question is always a specific one: which guna is running this decision, right now?
I've started asking myself that before important decisions. So far, the uncomfortable answers have been the most useful ones.