Origins of Grindelwald and Dumbledore

Gellert and Albus

 Grindelwald gets expelled from Durmstrang at the age of 16, because of twisted, dark experiments and near-fatal attacks on students, he also carves the deathly hallows onto a wall in Durmstrang.

After that he decides to visit his great aunt Bathilda Bagshot in Godric's Hallow. Actually he does that only to come to the place, because of the grave of Ignotus Peverell, who was believed to be the youngest of the three brothers, owning the invisibility cloak.

In Godric's Hallow he meets Albus Dumbledore, a neighbour of his great aunt. He meets him, while he lives with his younger brother, who is still attending Hogwarts at that time, and his younger sister, who is traumatised from past events, and has to be kept quiet and calm, or else she would turn into an Obscurial.

Gellert finds out, that Albus was supposed to go on a long trip with his fellow classmate and sidekick Doge, but his mother tragically passed away, after one of Ariana's outbursts (At that time his father was in Azkaban for killing three muggle boys, who attacked Ariana, when she was little. They saw her doing magic, at after she couldn't show them the trick again, they did horrible things to her, from that time on Ariana couldn't use her magic knowingly, and an Obscurial formed in her.) So they soon become very good friends (even though Albus feels they can be more than friends, while, as Rowling states in one of her comments- Albus is actually gay), and start making out plans for them leading wizards to becoming rulers above the muggle community. They both show huge interest in the Deathly Hallows and in the tale of the three brothers. They are certain, that the items are real, and they both show huge interest especially in the resurrection stone (which is the best for Albus, because he needs to get his mother back, to protect his sister, so he can go for his adventures with Gellert) and the elder wand (which is the best for Gellert, because he seeks it's power), also the invisibility cloak, but that only because they need them all together, to make them the masters of death (which I find interesting, that they, two really intelligent boys, would think, because as mr Ollivander specifically says " Together they make ONE master of death").

They even start writing to each other in the middle of the night, while they can't wait to the morning with their ides. One of the letters that Albus sends to Gellert states the following:

Gellert,

Your point about wizard domination being FOR THE GREATER GOOD- this, I think is the crucial point. Yes, we have been given power and, yes, that power gives us the right to rule, but it also gives us responsibilities over the ruled. We must stress this point, it will be the foundation stone upon which we build. Where we are opposed, as we surely will be, this must be the basis of all our counter arguments. We seize control FOR THE GREATER GOOD.

And from this it follows that where we meet resistance, we must use only the force that is necessary, and no more.

(This was your mistake at Durmstrang! But I do not complain, because if you had not been expelled, we would never have met.)

Albus

I myself find this letter fairly interesting, because in Harry Potter we see the exact opposite of Dumbledore- standing up for muggle rights and in this letter it shows Dumbledore wanting to rule over muggles.

Later on, when Albus and Gellert decide to leave Godric's Hallow, to go out and give speeches, to convince other witches and wizards, about them being right, and that wizard have the right to rule over the muggle world, Aberforth- Albus's brother finds out, about what they are about to do, and disagrees. Than there is a duel between Aberforth, Albus and Gellert, which Ariana is watching, and than while spells are flying everywhere, Ariana is sent to the ground, laying, dead.

Neither of them ever gets to know, who killed young Ariana. Dumbledore is left depressed, from the thought, that maybe it was him, who killed young Ariana.

The next day Gellert leaves, to fulfil his and Albus's plans, Aberforth leaves for school, and Albus is left alone to bury Ariana's body.

After that Grindelwald ends up using Albus's slogan "FOR THE GREATER GOOD" as his own.

I know this was very short, even though it took me like 2,5 hours, but I hope you now understand the origins of Gellert Grindelwald's behaviour in Fantastic Beasts. For me this is a really interesting part of the the wizarding history, especially on Dumbledore's side, but I guess we will be getting more information on this topic from J.K.Rowling in the later Fantastic Beasts movies. Anyway I ship Gellert and Albus so much, that I don't know anyone who would more, but come on, I'm such a Luna. I just think they'd look so cute together.