The most selfish
1 letter: I
Avoid it.

Most satisfactory 2 letters: WE
Use it.

Most poisonous
3 letters: EGO
Kill it.

Most used
4 letters: LOVE
Value it.

Most pleasing
5 letters: SMILE
Keep it.

Fastest spreading 6 letters: RUMORS
Ignore it.

Most coveted
7 letters: SUCCESS
Achieve it.

Most enviable
8 letters: JEALOUSY
Distance it.

Most essential
9 letters:
PRINCIPLE
Have it.

Most divine
10 letters:
FRIENDSHIP
Maintain it.