A broker will encourage you to use high leverage, but the reason is because your position size dictates the spread income. The larger the position, the more money your broker makes.
You wrote: Shouldn't I leverage heavily so I can earn more?"
Leverage means you want to gear (or borrow) funds to use for investing. If you borrow funds, you have to pay them back. This means you've increased your repayments - not your income necessarily.
If you're gearing for, say, a margin loan for buying shares or for equity trading or foreign exchange trading, then your interest rate will be considerably higher than a regular mortgage. More repayments.
If you're gearing for, say, an investment property to earn rental income, then you'd better be sure you've done your sums because not many rental properties are positively geared these days, meaning more money out of your pocket to cover repayments.
So.... how were you planning to 'earn more' buy borrowing money to use as leverage?
Don't ever try it. It will land you in hot water. When the time is good, you will think why others are so silly not to get more money to earn more money. But when the bad time come, you will be deeply regretted of getting into heavy leverage in the first place.
