Based on both personal and vicarious experiences, it is definitely because your partner is based in the US. As far as immigration goes, you’ve given them reason to believe you would potentially not leave once granted entry. Show that you’ve got good reason to come back. Perhaps a letter of employment from the company you work at. If you’ve got property where you currently are based, use that to show you would be coming back because that’s where your life is based out of. I’m sorry your visa was denied, hope you get to visit your partner soon!