-십시오 sounds formal, stern in speech. Things like -ㅂ니다, -시오 never carry your interest in getting long with the listener. For this reason customers feel like they are being impolite when the staff speak using formal forms.

whats more polite 십시요 or 세요 세요 is commonly used when you ask people to do something but if you wanna make it more polite, you can use (해)주세요. for ex; 하세요 => 해주세요 (do this please) 가세요 => 가주세요 (go please) and actually 십시요 sounds a literary style. maybe the difference between 기뻐요 and 행복해요 is like the difference between 'i am glad/pleased' and 'i am happy' :D ??