"I just don't feel like myself without you," is probably something you've thought when you've missed someone you care about.
This idea randomly popped into my head as I sat alone in my dorm missing my partner. I am not entirely sure if the science behind this is accurate because even the scientists aren't sure about the accuracy. Emotions are difficult to understand, and neurotransmitters are difficult to track.
So with that said, here's my take on things: a layman's definition of why you might actually feel like a different person when you're away from your significant other (or anyone close to your heart).
Biology and psychology teach us that our bodies naturally produce certain chemicals- hormones are produced by glands, and neurotransmitters by the central nervous system. Evolutionarily these chemicals help us to form emotional bonds to be able to maintain group relationships, intimate relationships, and parental relationships. They help keep us alive. Today, there's a lot more added to the mix, and as a result there's a lot more grey area.
The hormones related to "love" are estrogen/testosterone, and oxytocin. The neurotransmitters most closely involved are seratonin and dopamine.
Again, we produce all of these chemicals naturally, but when you are with someone you love, they surge. When they surge, your body speeds up to process them all. When you spend an extended period of time with someone you love, you basically become addicted to an elevated level of all of these chemicals, and your body becomes used to processing them all more quickly.
If your body is used to producing all of those chemicals, and processing them quickly, can you imagine what happens when you leave the person that causes it? In short, withdrawal happens. Your body stops producing an abundance of seratonin, oxytocin, etc., and to make matters worse, the chemicals that your body does produce continue to be processed so quickly it's as if they were never there.
Now you might be wondering, how does this impact one's emotional state? Well, in many ways, but it usually mimics symptoms of depression and anxiety. This is why so many people say, "I don't feel like myself," or, "I miss my other half," because their body has become used to certain stimulation that they are no longer receiving.
If you think about it, that's why the honeymoon phase in a romantic relationship feels like such a high at the beginning. Because that surge is new, and it feels good. They're all happy chemicals after all. But just like any drug, your body gets used to it, and it still feels good, you just might need extra every once in a while (hello date night).
Anyway, when you're ripped from the person that you love, it hurts. It could take months for your body to get back to normal, and every time you see that person in between, the clock is reset.
So before you beat yourself up for missing someone so much, remember this: you can't help it.