Why would someone always blame shift when I accuse them of something?

I've been seeing a guy for a few months now and have recently been feeling like he only uses me to get over his ex and to get sex. When I told him that I feel like he is using me he got mad, told me I should see a therapist and told me he felt like I was using him (which doesn’t make sense in my opinion because he has nothing I could get out of him). Why does he always play the blame shifting game instead of taking a stance to my accusation?

Why would someone always blame shift when I accuse them of something?
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