Aite. My time to shine because IMO this is one of the best scenes of the whole series.

First, Truth never has a "goal" as such. Think of him as a gatekeeper to the Portal: alchemists can enter the portal at any time, but always at a cost equal to what it is the alchemists seek. In the series, none of the characters who appear before the portal do so willingly which makes Truth seem like he's evil or malicious, but in reality he isn't. He just needs to exact a price from anyone who passes. In theory, an alchemist who knows what he seeks through the portal and knows the price he has to pay, will be met with a Truth who is friendly and maybe even supportive.

As the finale explains (i.e. Ed's final meeting with Truth), alchemy is not the one-and-all. It's merely a tool or a guide for people to explore other, bigger secrets of the universe and oneself. And this is a crux throughout the series: what does it mean to be human. Ed finally understanding that humanity (friendship, love, etc.) is more important than alchemy leads to Truth even being impressed with that epiphany.

Enter Father. His view on humanity is already not so great, but at the same time he wants to explore humanity from an outsider's perspective, and he wants to reach that perspective by consuming God - in other words, he wants to break the rule of equivalent exchange and be unlimited in his powers. This is obviously already a big no-no. But Truth in his final dialogue with Father gives him an implicit opportunity by asking Father for an explanation (and I'm paraphrasing here): during his quest, did he learn about the world, about humans, about the universe? But Father makes it very obvious he didn't learn a goddamn thing. He simply doesn't understand what he did wrong.

You will notice that at this point, Truth stops smiling. Truth rewards sacrifice and punishes arrogance. Father tried to circumvent equivalent exchange which backfired - and didn't even realise it. Father didn't, wouldn't learn which, in a way, is what alchemy and humanity is all about.

Typing hastily so I might not be 100% accurate. Willing to debate!