Praise be to Allah.
That is not considered to be bid’ah, rather that is one of the things that are prescribed in Islam because it is a means of achieving something that is prescribed, namely increasing love and friendship among the Muslims.
Allaah reminded His Prophet Muhammad (peace and blessings of Allaah be upon him) of the blessing of creating harmony among the believers, which indicates that this is one of the greatest blessings bestowed upon this ummah.
Allaah says (interpretation of the meaning):
“He it is Who has supported you with His Help and with the believers.
And He has united their (i.e. believers’) hearts. If you had spent all that is in the earth, you could not have united their hearts, but Allaah has united them. Certainly He is All-Mighty, All-Wise”
And the Prophet (peace and blessings of Allaah be upon him) said: “Be, O slaves of Allaah, brothers.” Narrated by al-Bukhaari, 6064; Muslim, 2563.
Increasing the love and friendship among the Muslims is one of the things encouraged by Islam, and it has prescribed the means of achieving that, such as giving the greeting of salaam, smiling at one’s fellow-Muslim, having a good attitude and giving gifts… and other things that are encouraged in Islam.
Anything that is a means of achieving love amongst the Muslims and increasing it is something that is prescribed.
And Allaah knows best.