If you don’t love yourself first, saying “I love you” to someone else is the equivalent of saying “I need you to feel loved”. Every human needs love and if your love is tied up in someone else being around then you absolutely run the risk of losing it. Even if the relationship lasts until the day they die. Morbid thought, but actually a pretty practical one too.
If your love comes from yourself first, then you will never be left alone – you will never find yourself out of love. You will never feel completely broken, for your wholeness comes from within, not from needing someone else to feel it.
This doesn’t mean life and relationships won’t won’t hurt you (they’re how we grow, that bit is kinda unavoidable) but it does mean that you won’t break into a million pieces with no idea how to put them back together if you love yourself first.