Everyone experiences negative thoughts or worries from time to time. This is normal. If these types of thoughts occur frequently and in many areas of your life, then it can become problematic and feed chronic anxiety. Most worries are focused on potential future events and the expectation for negative outcomes or worst case scenarios. Sometimes people will focus on “What Ifs” which are various hypothetical future scenarios where an individual conjures all types of bad thing happening (e.g., what if I get sick, what if I lose my job, what if I can't find a mate, etc.).
Uncertainty can also feed worry (belief that uncertain situations will have negative outcomes and that they'll be unable to cope). This can lead to different types of behaviors to reduce that uncertainty (e.g., Avoidance). For example, if you are experiencing a new pain and are worried about it, you may avoid going to the doctor because you fear the pain may be indicative of a serious illness. Procrastination can be another type of avoidance where postponing allows a person to put the uncertainty out of their mind (at least temporarily). There are also Proactive methods to reduce uncertainty such as looking for reassurance from others, trying to do most things on your own without assistance, or engaging in excessive, exhaustive research before making a decision. These behaviors are often related to feeling a Lack of Control with compensatory behaviors to produce an increased perceived sense of control.
Finally, Feeling Overwhelmed with too much to do and not enough time to complete everything can contribute to worry. Also, having too many choices to choose from can cause one to feel overwhelmed.