In my head the difference is that loving someone is a true love, unconditional, forever, whereas being "in love" is more transient like an infatuation or lust that comes at the start of a new relationship that can turn into a deeper true love in time or fade away to a friendship or even nothing. I think that is why some of the best marriages are between people who were friends before lovers. When someone says the phrase "in love" I think of the honeymoon period of a relationship, rom-com style love.