When somebody says 'I love you' to anybody, it could mean a variety of things. Either, 'I love you as a brother/sister', 'I really like you as a person', or even 'You just did something really funny, so I'll praise you by telling you I love you'
But for the most part, somebody will tell someone else 'I love you' because you have feelings for them. Not just butterfly-in-my-tummy feelings, or whenever-you-say-hello-to-me,-my-face-turns-bright-red-feelings.
When you tell someone you love them (as in, I have feelings for you), it means that you'd be willing to have a family with them. Be willing to have children with them. Get married someday.