My girlfriend and I are fairly young and you may be tempted to not take this seriously, but please do. We are very mature for our age and are involved with numerous things and activities that require us to be so.
Some background; I myself am 18, while she is 16. We’ve been together for six months now and have had a very steady and serious relationship up until very recently. She brought to my attention that to her, she feels like the relationship is slightly different for her now. She says that she still loves me the same, and still reciprocates everything, from my kisses, to our cuddles. But, she has also said that to her it feels as if the spark for her has gone out? Like I said earlier, everything appears to be the same for the most part, except for ourselves at times because we are now both aware of how she is feeling. I have a theory for this that has been confirmed by others, and that is that she is simply settling into the relationship and getting used to things. I feel that this theory is very accurate, because she has also told me that things that used to give her butterflies when I did them doesn’t have the same effect anymore.
Do you think that this is actually a relationship-ending feeling, or is it just a natural thing after being with someone for quite a bit? Also, we have talked and decided to give ourselves a while and see what happens.
LemonVibe 
You are both correct. I know that’s not what you want to hear.
The initial drive of any relationship tends to end pretty quickly after 3 to 6 months because its driven by hormones in your brain called oxytocin and dopamine that are very similar to how cocaine effects the brain and also why you stay awake at night thinking of them and feel the butterflies in your stomach making you nervous and not wanting to eat as much at the same time happy.
After that the spark has to do with if you are actually compatible on staying together. She’s actually correct in if she doesn’t feel it then you two may not be right for each other. That doesn’t mean you two aren’t good people. I do think you are mature for your age for discussing it at all. Relationships take the work of two people to work out. That means fights, discussions, one person doing 90% when the other can’t like in depression, job loss, deaths, hospitalization. You two have much, much more to discuss.
Best of luck to you. It’s not a hopeless situation. Just a huge uphill battle. I know one couple who did get married in high school and is still 18 years later, happily, with two kids. It was a lot of sacrifice and hard work on both parts, but it worked out for them.