Two people, no matter how close, are not the same as one. This is very tricky and subtle. There is no cause for dismay, but just another normally unrecognized facet of human life that has great impact on our internal and external existence.
Note what specific ways you change when brought into contact with another. Note along what lines your mind was wandering while alone, then specifically in what ways this changed when another person joined you. Note what seemed to be your emotional condition alone, then how your feelings shift, with the appearance of a companion.
At times, also note a definite change in your body’s condition, a tensing perhaps, when your solitude was broken by the arrival of another. You may also easily note changes in observable behavior.
Perhaps while alone you were singing and grinning, and when the other person entered the room, you without any conscious decision, immediately stopped smiling. Note it, observe it, and consider and Remember it. It is a new crack into which you may peek.
Once you clearly see this regarding your own condition and behavior, then consider the self-directed inquiry: “Why?” “Why is this so?”