Is there a reason behind spontaneous nipple hard-ons? Lately one of my nipples just stays that way for the majority of the day. It's irritating! Is there a reason behind it?
Yes, it can be irritating, and embarrassing, when nipples poke their lil' selves through t-shirts, sweaters, and/or padded bras, even layers of clothing, for all the world to see, and overstaying their welcome at times, too. What makes these teeny twins (or just one of them) stand at attention? Well, the skin of the nipple is rich in a supply of special nerves that are sensitive to certain stimuli. When stimulated, the nerves send a message to the smooth muscle in that area to contract. When the muscle contracts, the nipple gets tighter, producing a nipple "hard-on." The stimulus can be physical (a loosely fitting shirt that moves back and forth over the nipple), or it can be a drop in temperature. It can even be a psychological stimulus, like thinking about something sensual or sexual. These special nerves are part of the autonomic nervous system, which is instrumental in helping the body prepare for sexual activity.
Whether or not one has erect nipples also depends on the nipple holder. Some people experience them more often than others, just as some men have erect penises more frequently than other men. Levels of various hormones can also play a role, as they have effects on the autonomic nervous system, which is the part of the nervous system that controls the involuntary functions of internal body processes, such as heart rate and blood pressure.
You mention that only one of your nipples stays popped out longer than the other. If there's no discharge and discomfort, it's likely a random occurrence, one of those unexplained things our bodies do sometimes. Another much less likely possibility is an endocrine problem that could be causing a blockage of sweat or some other fluid at the sight of the nipple, which would need to be looked at by a health care provider.
If protruding nipples pose a persistent problem, wearing slightly warmer clothing or a firmer, more supportive undergarments could help. Perhaps bandaging the nipples as flat as possible might be another option — note: this can irritate the sensitive skin in that area. Relaxing yourself through meditation or yoga, for example, may also do the trick. States of deep relaxation can allow some measure of control over autonomic nervous system responses, such as the one that causes nipples to become "hard."