Tone and how you say it?

Then it's something you must work on or potentially lose your job. Practice by yourself or with a trusted friend saying things. Ask your family and friends if they have noticed this in you. Are you a typically outspoken and bold person? Sometimes these types of people are taken for blazen, bold and sarcastic with a bad tone when they are being themselves. But when it comes to a work place and being sensitive to your customers it is very important to watch how we say things.
I wish you all the best in fixing this.

Your tone of voice, facial expressions and body language play a role in how people respond to you. If you say things with an arrogant attitude, it can offend people. If your job is to deal with the public, then you need to work on being respectful and making sure you say things in a positive, upbeat way. Always smile and look the customer in the eyes.... listen to what the customer is saying to you and show them respect. If you are mad or having a bad day, you need to put that in your pocket and be kind and understanding.

Smile more, and always make sure that you smile when you talk - it will change the pitch of your voice. Even if you're not happy: just fake it. You may also want to try to make the pitch of your voice a little higher, especially giving a slight upward inflection at the end of each sentence - this will make everything you say sound a little more like a question, and therefore make you seem a bit more insecure (and less intimidating). :) Don't do it to the point that it comes across as patronizing, but just be aware that a smile and higher voice carry less authority and therefore appear less threatening. Good luck - you may get a friendlier reaction from some customers, but don't be surprised of others suddenly start trying to dominate you. It's human nature.

Try handing out balloon animals to customers.

Right, and once a month I have an excuse!

make a conscious effort to speak softer. watch for sarcasm in your tone.