In visibly our sentiments and feelings grow by time and attachments. I got personal experience about it today. I sold some of my parrots ( due to over load) today in bird's market. I don't know why but I am feeling sad after that even I have some remaining in my orchard. I think my attachment with them putting some emotional involvement in sadness. Are you feel such situations??
Yes the attachment is very common and a part of human nature! Take care of your birds!