Surrounding your body is an intelligent, defensive, energy shield. The Norse called it Hamr (pronounced similar to "hammer") and the Saxons called it Hama. This shield protects you from the dangers that lurk at the interfaces between our Middle World and the Worlds around it. It also defends you against the energies of other people.
In modern terms, this is what is seen by many as an aura. It has its own spiritual energy and acts as a connection between the physical body and the Wyrd. The exact strength and character of the Hama is dependent on the overall nature of the person.
You may have encountered people who just lit up every room they entered. You may have also encountered people who drained the energy out of everything and everyone. These are both effects of the Hama. A healthy, wholesome, energetic Hama makes things better. These people make plants healthier and animals seem attracted to them.
On the other side, an unwholesome Hama can cause psychic damage to all in the vicinity. Note that just being unwholesome does not imply weak. A negative Hama just seems to "vibrate" in a destructive manner.
Most of fit somewhere in between those two extremes.
Strength and weakness of the Hama has a lot to do with the overall condition of the person. If the person is otherwise healthy, he or she can more easily devote energy to the Hama and have a better psychic defense against the difficulties of the world. Likewise, when you are drained in otherways, it is harder to feed the Hama. General poor health can weaken the Hama, but it won't necessarily do so if the person has a healthy will.
Other things that can weaken the Hama have to do with bad habits. Abuse of drugs and alcohol can weaken the Hama, as can a sedentary lifestyle. Regular dishonesty or hostility to those around you can leave your Hama weak.
Since it is the job of your Hama to defend you, being under constant attack can weaken the Hama from the outside. If you are around negative or hostile people all the time, they will wear on your energies and your Hama will suffer.
Naturally, we are all complex creatures and probably cannot remove all negativity from our lives. How we deal with the negativity will determine whether we feed our Hama and what character it takes.