The reasons you should always last hit are:
- It's faster to last hit a bunch of minions by only attacking them once and getting the gold than hacking away at one minion at a time for the gold.
- It saves you health from being in the minion fray as you just wait on the side for a minion to drop to the right health.
- It keeps your lane pushed back which is always a good thing for a couple reasons. You'll get tower support during fights. You won't get ganked from behind like you would if you were pushed to the enemies tower. It gives you a lot of room to kill your enemies before they can run back to their turret. It gives space for your jungle/mid to come down and gank your enemies
Last hitting comes with practice. It's like a natural skill that comes from the right timing. You'll get used to the timing over time (haha get it? ;]) You'll see how different characters attack animations and attack speeds are and you'll get used to running up to a creep when the creep has the right amount of health at the right time to get the profit.
Practice last hitting! It is a required skill to properly lane safely with the most amount of gold.