About のこと

The こと is used in two ways when referring to a person.

• First, it is less direct. If you've studied Japanese for any length of time, you'll know Japanese love to go indirect. It's more ambiguous but also more polite and maybe more emotional too. That's maybe why it is often used with words like 好(す)き (to like) and 嫌(きら)い (to dislike) and 思(おも)う (to think).
• Second, and most importantly, the こと implies everything about the person. The person's appearance, behavior, lifestyle, personality, etc. So 「君(きみ)のことが好(す)き」 means you like everything about that person. And 「君(きみ)が好(す)き」 just means you like the person.

君(きみ)のことが好(す)き。

I like you

This implies you like everything about that person: his or her appearance, behavior, lifestyle, personality, etc.

君(きみ)が好(す)き, on the hand would just mean you like that person.

In other words, the のこと is more specific and therefore isn't used any old time.

Only use こと when referring to a person

1. to sound more indirect
2. to include everything about that person.

君(きみ)に話(はな)したい。

I want to talk to you.

Since the sentence is unrelated to the person's appearance, behavior, lifestyle, or personality, don't use こと. Now, how about this one?

先生(せんせい)にプレセントをあげた。

I gave the teacher a present.

You are stating the fact that you gave a present the teacher. The teacher's appearance, behavior, lifestyle, or personality is irrelevant.

先生(せんせい)を尊敬(そんけい)しています。

I respect my teacher.

In this case, you respect everything about the teacher, so you can also say:

先生(せんせい)のことを尊敬(そんけい)しています。

I respect my teacher.