I think it is a good quote of advice to all to take proper caution on how you love someone but i think it is not entirely true. Love is not really something we can control or moderate. I believe we don't even get to choose who we love. We see people who never thought of falling in love falling in love and so much in love, some people fell in love with people they never expected. So you see, it just happens and how deeply we love is not of our choosing actually.
Love is a risk we choose to take knowing it can bring us great pain and hurt but still we choose to love despite knowing all these because we have that glimpse of hope of finding that true one and having a fairytale end. You are either fully in or out. There is no such thing as loving but not too deeply. Then, to me to be honest; you are not in love to begin with. Because when you love someone, you love them with your whole being, body and soul and if that is not deeply then i don't understand what it means to love deeply.
The quote tries to comfort those who are scared that they might be hurt really bad if they fall in love but it is not something we should be scared of. Love sometimes hurt and a quote will never change that or prevent that. We must be ready to face it as it comes and let go of our fears and comfort ourselves with the hope of finding that one and only true one for us.