Why do people like greifing? Here are a few reasons:

For fun:
- It entertains them. So they raid peoples bases for no apparent reason.

Loot:
Greifer: Im tired of mining. Hey there's a base full of gold and other loot over there!! Why mine for ores when I can break some chests???

War:
Many times, they don't even want what you got. But when you anger them, expect a raid sometime.

Builder's a N00B:
If you build your spawn like 100 blocks from base do these steps:
1. Get a stopwatch
2. See how long it takes for you to be raided
3. If it has a player average of 50+, probably within 6 hours or so.
Yea, build it far.

Beacons:
Greifer: Where can I find a base... A BEACON!! That means...RAIDING TIME!!!!

Betrayal:
Sometimes (it's sad I know) people will backstab you and raid you for your valuables.

What to do to prevent this:
- Have a super secret base revealed to NO ONE.
- Try not to have all your valuables in one area.
- Do not have above ground or easy-to-find bases
- Try using Obsidian or lava to block the base (use this for underground)