Why does dispell remove friendly buffs?
 

I hate that I end up removing stone skin when I am trying to dispell curses :[

Why?

as far as I remember, at level 3 you no longer have this problem.

Yes, at level 3 it only removes negative effects from friendly units.

One of the main reasons for lvl3 order magic :D

It says so in spell description :
1 lvl - remove all buff from friendly unit
2 lvl - remove buffs from friendly or enemy unit
3 lvl - remove negative buff from friendly units, or positive buff from enemy unit.
So to keep positive buffs on your units, get it to 3 lvl.

Besides the goblin shaman spell "Leech of life" or something like this:-(

There are a few times you need to use Dispel rank 1 or 2. Some effects are considered unfriendly that are, and vice versa. For instance any 'domination' effect will be considered unfriendly, but the target will be considered friendly to its new orientation. So if your stack is dominated, it will be an enemy. And level 3 dispel removes GOOD things from BAD people. But the effect is considered a BAD thing on a BAD person and so is unaffected.

You can dispel it on an enemy troop that has at least one other buff/debuff. It just does not show up as a viable target if it has only threads of life on it. So a workaround is to cast an auxiliary debuff at the enemy unit and the dispel it (not sure about dispel lvl3, it works with lvl2).

Nice tip! I thought about dispelling it from the enemy but he didn't have a debuff!

Once again, orcs manage to be a pain in the butt... :)

Yes, it still is a pain in the butt... probably even more so if you are not playing a soothsayer - it's more reasonable if you are able to cast both spells in one round, as long as the fight is not so hard that you have to cast other vital spells. But then again, thread of life seems to be supposed to bind friendly units together, so it will hopefully get patched sooner or later.