Eye contact is a very individual thing. For some people, if you look too long then it can come across as being too intense while if your eye contact is weak then you can come across as being a bit dodgy.
I remember when I was in intermediate school and I was in I think it was cooking class and I was in one of my daydreams but happened to be looking at the cutest girl in class. I wasn't looking at her per se but thinking about something totally unrelated to cooking class and the girl. To her however, I was staring at her and she was not comfortable and I got the "if looks could kill" look and she said to me "Yes?" I had to come out of my trance. I didn't know what to say. I think I muttered sorry and looked the other way and concentrated on my (Spanish) omelette.
Sometimes intense eye contact can make you feel good because it means the other person is really listening to what you are saying. There are a couple of people I think of when I write that. I know they are listening because you can tell that they are consciously doing what they are doing. They have practiced it over many years.
Eye contact is important, there is no doubt about that but everyone has their own threshold and you have to be weary of that.
The problem is if you don't have a sufficient amount of eye contact, there is a possibility that you can lose out on some opportunities. For example your eye contact might be poor during a job interview and bang, that job has fallen by the wayside.
How can we improve our eye contact.
The cheat's way could be to focus on something other than the eyes. This would give the illusion of eye contact but you are felling more comfortable.
Remember though, you need to break it on occasions because just staring, is just plain rude.