Some countries have a word for love between friends. For example, in Korean, the word is called Jung. The English language doesn't have this word. Basically when you feel love for a friend, you care for them deeply. You should understand that when someone says 'I love you' between friends, it doesn't imply anything sexual.
Love between the husband and wife loses the initial bubbly feeling, but it becomes more powerful and more solid. It's easy to express how you feel by three simple words.
