Why is the Sermon on the Mount so intense?

My mother is upset there’s no marriage in Heaven. Why does Luke 6 read differently from Matthew 5 in terms of the killer sermon of Jesus? Is Paul actually teaching we’re not under the Law for salvation, but still under it for daily living? Is it selfish or sinful to save for retirement? Shouldn’t we follow the Sabbath? To support this ministry financially, visit: https://www.oneplace.com/donate/1258/29