I was re-watching the Marvel Phase 1 movies and I couldn't really understand Loki's motives for conquering Earth. During Thor he falls to his apparent death and in The Avengers we find out that he hasn't died but has been with Thanos who gives him his new sceptre and the Chitauri army to conquer the planet.
But what is Loki's plan? Isn't he after Asgard and Odin's throne, and how does Earth fit in all of this?
There are multiple reasons for why Loki attacked Earth.
The first and most obvious motive is also the simplest - Revenge.
From Avengers Assemble;
Thor: We were raised together, we played together, we fought together. Do you remember none of that?
Loki: I remember a shadow, living in the shade of your greatness. I remember you tossing me into an abyss, I who was and should be king!
Thor: So you take the world I love as recompense for your imagined slights? No, the Earth is under MY protection, Loki!
Loki has always felt that he was over-looked by Odin in favour of Thor, the true born son of Odin and the first in line for the throne. He's jealous and angry, and he thinks that he would make a much better king than Thor would - so after Thor ruins his plan to rule Asgard (the events of Thor), he chooses to take revenge against his brother by attacking the world that Thor loves.
The second reason is that he was asked to.
A long-running but (as of yet) unresolved plot throughout the Marvel Cinematic Universe so far is that Thanos wants to unite the six Infinity Stones. One of these, the Space Gem, is stored inside the Tesseract, which the Red Skull used to power Hydra weapons in Captain America: The First Avenger and at the start of Avengers Assemble is being experimented on in SHIELD.
Thanos gifted Loki the sceptre that he wields throughout The Avengers in order to retrieve and return the Tesseract to him. In return, he gives Loki a Chitauri army with which to attack the planet. This is beneficial for both Thanos and Loki - Loki gains the ability to travel to Earth and an army with which to take it, and Thanos gets the Space Gem.
The third reason is a little more complicated and (as of yet) mostly guesswork - but Loki needed to return to Asgard.
After falling off the Bifrost Bridge, Loki had no real way to return to Asgard, the place that he wishes to rule. However, by attacking Earth (and losing the war), Thor takes him back to Asgard to be imprisoned in the dungeons. Loki later uses this imprisonment to his advantage, and as of the end of Thor: The Dark World actually manages to take control of Asgard without anyone knowing - becoming the King that he originally wanted to be way back in Thor.
How much of this was planned out in advance is still currently unknown, but knowing that Loki is the God of Mischief means that this could well be a very valid reason for why he attacked Earth - the first part in a much larger plan.
According to the junior novelisation for Avengers Assemble, Loki's motive in trying to take over Midgard was ... erm ... so that he could take over Midgard.
Loki was pleased. Thor and Odin thought him dead. He’d tricked them when he fell from the Bifrost. They would not be interfering with his plan. He’d find a way to send himself, body and soul, back to Midgard, and once he was there, he would rule these simple mortals, in a way he wasn’t able to rule Asgard.
A Realm would be his at last — just as he deserved.
I would like to add to Dr. R Drizzle's good answer with the following Loki quotes:
Kneel before me. I said, KNEEL! [Loki stamps his scepter on the ground, causing a shockwave that intimidates the crowd into silence as they all kneel before him] Is not this simpler? Is this not your natural state? It's the unspoken truth of humanity, that you crave subjugation. The bright lure of freedom diminishes your life's joy in a mad scramble for power, for identity. You were made to be ruled. In the end, you will always kneel.
And the memorable scene at the end of the movie before the Hulk rag-dolls Loki:
Enough! You are, all of you are beneath me! I am a god, you dull creature, and I will not be bullied by... [Hulk smash]
Loki is a god. As such he demands worship and believes that dominating earth is simply the natural order of things.
Loki can be heard saying in Thor and in other films, plus the Loki series, that "he never wanted a throne." In Thor, Loki says that he only ever wanted to be Thor's equal, and in the Loki series, he tells Mobius that his actions were "an illusion conjured by the weak to instill fear, a desperate play for control."
My thoughts on why he acted out as a villain in the first Thor movie is exactly why he says he did it at the end, when he falls off the bridge. He wanted to prove to Odin that even though he is the son of Odin's former enemy, he is still worthy of being Thor's equal, to be treated with the kind of care Thor is. He chose to destroy his own race of Frost Giants in order to show that he's not one of them, that Odin doesn't have to fear him, that he isn't "the monster adults talk about in children's bedtime stories."
I think that his actions in The Avengers movie and everywhere else we see him being the villain are for this same very reason, to prove that he can do something great or "glorious." His "purpose" for doing these things that cause others harm is to show that he isn't weak and he is strong and worthy enough to stand in the same places as Thor and be seen as worthy by Odin, that he deserves his praise.
However, throughout the MCU, as things happen, not all of them might've been according to his plans, and some minor inconveniences for what he wants, there might've been a few moments where Loki had to improvise and just did what he thought would help him gain control over the situation. I do think that in the first Avengers movie, he could've been working for Thanos at that time and tried to conquer Earth for his own personal gain with Odin, as well as gain the Tesseract for Thanos.
By watching the Loki series, we can see his reactions to everything he's done in the past, and we get a look inside why he did all of it through his conversation with Mobius. Overall, I don't think Loki is a villain at heart, as all he ever wanted was to be seen as worthy of being Thor's equal and of Odin's approval and praise. Loki wasn't able to pick up the hammer in the movie Thor because Odin never saw him as worthy of great power like that.
