Let me change your question into "When do we drop '요(yo)' and when do we use '요' and 'ㅂ니다(mida)'?" because this is exactly what you're asking about. If you use 요 with someone, you show basic respect for the listner because you don't know them very well or because they're older than you. You can drop it if they say you can, or you can ask them. If you drop 요, you emotionally open up yourself to them. -ㅂ니다 is a formal verb ending, which is rarely used in day-to-day conversation.

saranghae is imformal way to say 'I love you' and saranghaeyo is formal way. And you can say saranghamida when you say 'I love you' to a lot of people.