Does passion always die in relationships after the first few years? Can it ever be rekindled?

I am no expert by any stretch but I have been married for over 15 years and experienced what you are talking about.  It is true that the brain does produce the chemicals that give you that dizzy euphoric feeling when you first fall in love with someone and like any other drug you will develop a tolerance for it plus your brain will start to produce less of them.  If you want to ignite the spark again or make sure that when things die down you can ignite it, first you must always treat each other with respect. 
When you don't you will cause emotional wounds in your partner which will cause you to not have the tools to get the spark back.  Then the key is novelty.  Novelty in anything.  It could be in sharing new food or it could be in exploring a new place together or it could be in learning something new together, not to mention, trying new sexual adventures.  These things will be like when you first got together and everything was new for the two of you as a couple. It can recreate the excitement of not knowing the future and leaving the possibilities open.  Even though it can seem like you know your partner oh so very well, you must remember that all of us have all kinds of aspects to our personalities that are not always expressed but just under certain circumstances and unless you have seen them in all possible circumstances you don't really know what they may be like.  Remember then that there is so much more you have to learn about them encourage them to grow and allow them to become as we are all becoming who we are always.  Treat them the way you want to be treated.  Don't take each other for granted and expect anything from them.  try to remember when you first got together how special each and every little thing they did for you was?  Recognize that those same things are just as special now.  For instance if your partner made dinner for you when you were dating and it felt good that they went to the effort for you but now they make dinner for you every night and you have come to actually expect it and even complain about it...well you have to lose that attitude and appreciate the effort just as much because they still did it for you and it took them just as much thought and energy now as it did then the only difference is in your attitude and the lack of novelty in it.  So remember they are still paying the same price to do it but you are the one devaluating it.  Be appreciative of them. Don't compare them to anyone else.  Also if you think they have changed and you don't like who they are becoming remember that you have had a part in shaping who it is they are now.  Be willing to be firm in your boundries but also be open with your feelings and when they try to communicate don't judge them with any of the things that hurt you from the history you have shared.  If you were just meeting them for the first time and didn't have any emotional investment you wouldn't hear them the same way you do now.  Try to just listen and then when they are done be sure to validate what they have said to you. Be honest with them. Don't forget to flirt with them through out the day/week.  It can be very seductive.  Last but not least protect your relationship and treat it as if it is the most important thing in the world to you.
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