Learn Farsi: او به دلیل اینکه به هدف خود دست یافت، بسیار خوشحال است. - Because he achieved his goal, he is very happy.

دست means hand, and یافتن means to find or obtain. Together, دست یافتن literally translates to to find hand, but idiomatically it means to achieve, attain, or gain access to. Inchoative verbs with یافتن indicate a process of acquiring a state or thing, making this a very formal and elegant way to say he reached his goal.

Why is the preposition به used before هدف (goal)?

The compound verb دست یافتن requires the preposition به to introduce the thing being achieved or accessed. Whenever you use this verb to say you achieved something, you must connect it to the object with به.

I usually use چون for because. Why does this sentence use به دلیل اینکه?

به دلیل اینکه literally means due to the fact that or for the reason that (دلیل is reason). It is much more formal than the conversational چون. Using compound conjunctions like this elevates your sentence to a more advanced, written register.

Could I use the possessive suffix and say هدفش instead of هدف خود?

Yes, هدفش (his/her goal) is grammatically correct and very common. However, using the reflexive pronoun خود (self) to say his own goal emphasizes that the goal belongs to the subject of the sentence (او). خود is frequently preferred in formal writing for stylistic reasons.