There are 16 verbs in French that require être rather than avoir in the passé compose. Sometimes there is a passé. Passé uses être to talk about physically passing by something. These irregular verbs also must agree in gender and in number.
To love Nature is to think of all the ways you can fall in love with Life, without that word referring only to you and your life. It is in our nature to love Nature - we have just forgotten how to fall.