The gemara says [Bava Basra 164b] "A person should not praise his friend because this might lead him to speak negatively about him." The Rashbam explains that EXCESSIVE praise leads to slander [people have trouble thinking that people are SO good so they like to find some "chinks in the armor"]. Rashi adds [Erchin 16a] that not only might the speaker add some negative characterization but the listeners might as well.
The Rambam [Deyos 7/4] understands the gemara to mean that one should not praise someone [at all] in front of his enemy. The enemy will likely hear the kind words and find some lashon hara to say.
Interestingly enough the Rambam [Deyos 6/3] says that ואהבת לרעך כמוך is fulfilled by praising another person.
It would emerge then that when one excessively praises his or her friend, according to the Rambam one is fulfilling a mitzva from the Torah of ואהבת לרעך כמוך while according to Rashi one is doing an aveira [because it might lead to lashon hara].
The Rambam also says that one should not publicly praise someone else because this will cause one of the listeners to say something insulting.
This topic is a broad one and we have just scratched the surface....
[See Likkutei Sichos Vol. 27 Parshas Emor and the Sefer "Bitzror Hachaim" Vol. 4 Page 175 and on]
