You know, my delusional mind tells me a few stories. I was going around looking for remedies and I encountered a few theory. i did my own investigation. Remember on 17th Jan when Taeyeon was very depressed and went out alone to the beach to think? Then one day Taeyeon said if you really love someone, you got to let it go? I believe taeny dated. BUT here comes the third party and fany swayed. Taeny may have dated for very long. Their relationship may have lasted for very long. They are together for almost 24/7 but they could not go public so a very bubbly and outgoing person like Tiffany cannot stand it. Then with yoona and Sooyoung dating guys and going out in public together, she started have this longing. Thinking that if she dates a guy, she can do what she always wants to do with her lover. Taeyeon may be laughing with Tiffany on shows, showing all the photos and all but if you observe, unless someone interacts with her on a show, she seldom laughs and blank out. And all of a sudden, she said she feels lonely. She may be putting up a facade. Because if Taeyeon had Tiffany, she should not feel lonely but because fany left Taeyeon, she feels a part of her left her and that explains the loneliness. Taeyeon is one who don't like others to worry about her, she tries to show her fans that nothing is wrong and also to show Tiffany that she is OK with the break up. She may be crying every night when Tiffany goes out for a date. She also became thinner. She became thinner, sad and depressed since Jan and it fits fany dating period. Just my theory.

Taeyeon became very lonely since last year and she drank her way through through nights. They have been quite on and off. We notice that there are a few fights between them last year. Remember when Taeyeon suddenly go out with heechul then after that Taeyeon post some kind of fighting picture with heechul? Then kangin said something about couple breaking up? Maybe Taeny already had relationship issues and Nichkhun appearance ticked fany off and after break up and fany going out with Nichkhun, Taeyeon went into depression mode.