Despite it's "love song" quality and apparent meaning, I get the impression at it's core it's a song dedicated/directed at her dog, now desceased, named Willow.
She's saying how she acts, flirts, "falls" for and pretends to be in love with men that are unattainable ("spoken for") and/or toys with men, yet never really retaining meaningful affection for them. But "Willow" is her only source of true love, whom she gives all her heart to and trusts the most.
Despite it's "love song" quality and apparent meaning, I get the impression at it's core it's a song dedicated/directed at her dog, now desceased, named Willow. She's saying how she acts, flirts, "falls" for and pretends to be in love with men that are unattainable ("spoken for") and/or toys with men, yet never really retaining meaningful affection for them. But "Willow" is her only source of true love, whom she gives all her heart to and trusts the most.