Why does she looks at me with sad eyes?

While I am leaving, I tell her goodbye, she looks at me with sad eyes. Kind of a stare with out emotions. She was not smiling or anything, she's a natural smily girl too.

I know I am a shy guy, it's look like she's trying to understand me from looking in my eyes. We had a disagreement in the past also, she seems to not have recovered fully from that. Altrough, she laugh a lot at my jokes. She is a cary person. She also said in the past i look depress...

i just want her to be confortable but she act strange by looking at my in sad way, why?
Why does she looks at me with sad eyes?
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