Paschendale, when i hear about an attraction or loving connexion "decreasing", i see how much social conditioning works still: somewhere we think to know that it is not allowed to love more than one and, what's more, many times humans feel guilty of feeling attracted or in love with more than one; so the previous (the first in terms of time) falls as humans "normally" seek for new, above all when very young -but not only. the components i'm talking about work usually "underground", as every old sticking conditioning.
i add: it seems that after one year and a half of relationship, the woman changes her attitude as some hormones and chemical substances giving the "being in love state" get lower; at that moment the "man" is requested to make his partner fall in love again, or she'll get some new falling in love with someone else.
it seems from the same research that men' falling in love works on/with other chemical stuff that makes them to fall in love for 20 years or more (but i do not agree so much with this last thing, especially if the man is B...)
take care and love yourself, everything is perfect indeed.