Why was slavery an issue for westward expansion?

Can you summarize this? it takes time and caution to perfect anything which must be concealed. it takes time to crush men who are pursuing legitimate trade. but one of mr. rockefeller’s most impressive characteristics is patience. there never was a more patient man, or one who could dare more while he waited. the folly of hurrying, the folly of discouragement, for one who would succeed, went hand in hand. everything must be ready before he acted, but while you wait you must prepare, must think, work. "you must put in, if you would take out." his instinct for the money opportunity in things was amazing, his perception of the value of seizing this or that particular invention, plant, market, was unerring.