Two drives run deep and conflicted in most of us. They go by different names, id and superego, yang and yin, assimilation and accommodation, the courage to change; the serenity to accept, "all about me" vs. "all about us." Here I'll recast them in practical terms. I'll call them doing well vs. doing right.
You care about both.
Doing well means, at minimum surviving; getting your needs met, and at maximum thriving, getting exactly what you want reliably, consistently and forever more.
Doing right means, at minimum, not doing evil, not harming others and at maximum being helpful, accommodating, tolerant, generous, even selfless.
It is often but not always possible to do both at once. Life gets interesting and confusing when the two goals are in conflict as they often are.
You'll also hear efforts to reconcile the two drives as though there's never any conflict between them.
In general how best to manage the tension between doing well and doing right? Not with some purist formula but instead with the "wisdom to know the difference" between situations that are best served by each approach.
So how tough should we be? How much should you look out for number one? It depends on whom you're dealing with. If you're dealing with a very tough indivudual or organization you had better be tough. If you're dealing with someone who cares about what's right, it's wrong to gun for what you want with your highest caliber bullets. Ideally, we find the right counter-pressure for the situation.
In reality, we tend to impose a little more counter-pressure than is needed. We do it because in the short run, running over others is less risky than being run over. In the long run, this leads to escalation, both sides focusing instead on doing well at whatever cost to what's right.