Never start a sentence with 'because'
There are many rules of grammar we are taught in childhood that stick with us when we become adults. We may not remember the why, but we remember the what.
But not all rules are really rules. Some are misremembered. Some mistaught. Others misapplied.
They stay in our minds and colour how we use language through life. How we use language incorrectly.
These are:
The Seven Sins of Syntax
The Sin
The word 'because' is a conjunction. A conjunction connects two separate words, phrases, or clauses in a sentence.
There are three types of conjunctions.
Coordinating:
Words that connect two things of equal importance: for, and, nor, but, or, yet, so (FANBOYS).
Correlative:
Pairs of words that show how two things relate to each other: eg, either/or, both/and, as/so.
Subordinating:
Words used to show the dependency of one thing upon another: eg, since, after, therefore and because.
As a subordinating conjunction, 'because' requires both a cause and an effect. If nothing comes before the word then logically, nothing can follow.
When a sentence begins with the word 'because', there is literally nothing before the word. This leads to us believing it's a sin to begin a sentence with 'because.'
Not So Sinful
The position of a word in a sentence does not define the role it plays. It is perfectly correct to begin a sentence with the word 'because', if the sentence still includes both clauses. You just need to flip the usual structure.
So instead of
I didn't go out because it was raining,
you can say
Because it was raining, I didn't go out.
Instead of
He didn't call because he hates her,
you can say
Because he hates her, he didn't call.
Committing More Sins
If your sentence stands alone, you shouldn't start a sentence with 'because' unless you are flipping the clauses.
However, within a paragraph, beginning a sentence with 'because' without two clauses at all is now commonly accepted. It can add emphasis to the subordinate clause which would otherwise be, well, subordinate.
For example, there is more implied in the reasoning here:
I didn't go out. Because it was raining.
He didn't call. Because he hates her.
Used sparingly, beginning a sentence with 'because' can effectively break the pace of your writing. Because it interrupts natural flow.
