In some cases, unfulfilled needs dominate my thought process. When I need something of importance, I tend to obsess on it to the point that it might become even more important than it need to be.
On the other side of it, once a need is fulfilled, I usually stop thinking about it. That leaves plenty of time to worry about the unfulfilled needs!
On a very basic level, when I am hungry I feel irritable and easily frustrated. When I am full I am usually content at best and sleepy at worst. Emotionally speaking, when my emotional needs are met I feel confident, and joyful; however, when they aren't met I feel sad and alone.
When my needs are fulfilled, I tend to be more pleasant than when they are not. If some need of mine is not being fulfilled, I tend to (whether I really want to or not) be somewhat grouchy. I can sometimes feel very disillusioned because I feel as if the world is not giving me the things that I need and deserve.
There are as many different ways to answer your question as there are people and needs.
When I needed more to eat earlier today, it affected my thoughts by making it difficult for me to concentrate on the task at hand. My behavior wasn't directly impacted - I kept on working on the project I had begun, but my thinking process wasn't completely focused on the work. After I fulfilled the need by getting a sandwich, I was able to devote all my attention to my work.
Hunger is a fairly straight-forward and easy to define need - we all need nourishment in order to survive. A need that is more difficult to measure, but still important, is a sense of personal esteem. Persons who do not feel loved and needed by others show clinically verifiable symptoms of depression, stress, and other unhealthy conditions due to their unfulfilled needs. Such individuals may think and act in ways that are shaped by their feelings of isolation, loneliness, desertion, uselessness, and so on. In students, this can lead to falling grades, poor attendance, apathy or rebellion toward school and authority figures, and possible use of substances that are perceived as providing a method of escape from the pain of the needs.