There could be several reasons why your best friend invited someone else to hang out with you when you had planned for it to be just the two of you. Here are some possible explanations:
Spontaneity: Your best friend might be a spontaneous person and enjoys making last-minute decisions to include others in social activities. They might not have thought it would be a big deal to bring someone along.
Avoiding Awkwardness: Sometimes, people feel more comfortable in social situations when they have others around. Perhaps your best friend thought that having another person there would help ease any potential awkwardness or tension.
Shared Interests: If the person your best friend invited shares common interests with both of you, they might have thought it would be enjoyable for everyone to hang out together.
Prior Commitments: Your best friend might have made plans with this other person before you suggested hanging out, and they didn't want to cancel on them.
Inclusion: Your friend might have wanted to include this other person to expand their social circle and create a more inclusive atmosphere.
Miscommunication: It's possible that there was a miscommunication or misunderstanding about the initial plans, and your best friend didn't realize it was meant to be just the two of you.
Avoiding Intensity: If you and your best friend have been spending a lot of one-on-one time together recently, they might have wanted to bring someone else along to lighten the intensity and have a more relaxed hangout.
Regardless of the reason, it's essential to communicate openly with your best friend about how you feel. If you had specific plans for a one-on-one hangout and feel disappointed or hurt that someone else was invited, let your friend know how you feel in a non-confrontational way. Honest communication can help avoid misunderstandings and strengthen your friendship. Additionally, try to be understanding and considerate of their perspective as well. They might not have intended to upset you and might be receptive to your feelings once they understand how you feel.