Why should an employee always smile to his/her manager?

It is not a thumb rule that employees have to smile. It is socially valuable to smile to anyone that you meet.  A Smile flexes the mind set to a positive frame and it opens up communication removing any reserved barriers.

1. Smile creates a positive feelings
2. Shows your boss that you are ready for any challenges given to him
3. Positive attitude towards work
4. Easy to be approached

A smile is needful even for the office boy or the driver, everybody.

Al-Ebtisama fe wajh a5eek, Sadaqa.

Smile shows happy and positive attitude, but it does not mean always supervisee will be smile face. Employee does smile face because:

1. Feel comfortable in work.

2. Love the work.

3. Enjoy the work.

4. Able to carryout more responsibility.

Smile show your attitude and confidence. smile show that u dont have big problem and u r not facing problem from your work and your senior or your manager. Smile is a big key for easily to talk to any one and make frank to every one.

A smiling face is always attractive . If you show your manager an angry face he will become more angry but if you smile he will feel good feeling about you. Better to make a beautiful house in his heart and don't try to destroy that because to impress some one is the most difficult job and specially a boss . So keep smiling and do your work with perfection. Smile is also a " Sadaqa" in Islam means( You give some one free for goodness to make him/her happy).Also you smile to your parents then it's bring more " sawab" mean ( Rewards from Allah) because when parents become old they need love, care and attention. A smile face increase the face value. :) So smile now :) That's like a good  person :) 

All of use need to know Sales and marketing skills, not only for external customers, but very important tu use these skills with internal customers (Peers, Subordinates, and Managers). Smile is a key factor for effective communication

I agree with all 

Because It’s a  beginning  always & etiquette too.

Smile? Or, do you mean cordial?

Smiling is not necessary when dealing with your manager or with anybody else, especially when it is uncalled for.  But to be cordial is to be courteous, not only to your manager but also to your every co-worker.  It should be a habit, not an obligation.