During the first year, when the NRE wears off.
I don't really believe that NRE "wears off". What happens is that during the first few months of infatuation, the relationship is mostly based on fantasy. As the couple gets to know each other, the fantasy is replaced with reality. If the attraction is lost at this point, especially within the first 6 months, it's pretty much over. The primary attraction just isn't there.
When the couple moves in together.
Moving in together requires a renegotiation of attention. When a couple is dating, they typically give each other their full attention when they spend time together. When living together, this isn't possible. You have other shit you need to do. This can lead to one person feeling neglected and ignored, and the other feeling demanded-upon and smothered. This kills sexual attraction.
When the couple gets engaged or married.
Marriage entails a big, ostensibly life-long, commitment. This can lead to feelings of being "trapped", ambivalence, and an urge to distance oneself or flee. On the flip side, a commitment can lead to complacency and feeling like it's not necessary to put in the effort anymore because the other person is "locked down". Especially if someone hasn't been enjoying sex, but only doing it to please their partner, they may feel that it's not necessary to continue once they've secured a commitment.
Pregnancy or the birth of a baby.
The transition to parenthood is difficult for most couples. For both partners, there is a loss of freedom, stress, and anxiety. The new mother typically feels exhausted and overwhelmed and dislikes her body. The new father typically is jealous of the baby (but can't admit it), is stressed over finances, and feels emotionally neglected. This is a recipe for resentment and conflict, which are huge sexual turn-offs.
Loss of a job, moving to a new city.
These are stressful events that also affect a person's sense of self-worth and social support system. This may cause neediness and over-reliance on the romantic partner for support, which kills attraction.
Physical or mental illness.
When one partner has to act as a caregiver for the other, it often leads to a lack of sexual attraction. The relationship may start to feel like more of a parent/child relationship than a relationship between lovers.
An affair.
Even when an affair is forgiven, it's not forgotten. Sometimes it's difficult or impossible to regain trust and attraction.
Fortunately, it’s possible to keep the passion alive with a little effort. Here are some tips for a Passionate Marriage
DON’T GIVE UP ON SEXUAL SATISFACTION
A dead bedroom isn’t a death sentence; it’s a temporary problem that can be solved if you put in the effort. It also can take time to rekindle the flames, so be patient.
COMMUNICATE EFFECTIVELY
Let your partner know that you’re not as sexually satisfied as you could be. Let your partner know when you’re in the mood. Let your partner know your likes and dislikes in bed. A lack of communication might as well be a brick wall separating you and your partner.
EXPRESS LOVE
Simple physical expressions of love, such as cuddling and kissing, are great for your relationship. In addition to making you and your partner feel loved, it can also help you feel friskier. Verbal expressions of love, including saying “I love you,” flirting with your partner, or sending a suggestive text message is useful in fanning the flames. However, it’s important to also express love outside of the bedroom. Acts of service, giving gifts and spending quality time together strengthen the bond between you two.
MAKE THE EFFORT TO SET THE MOOD
Life can be busy and stressful. Don’t let this stop you from making time to make love. Foreplay, romantic music, candles, and weekend getaways all help keep the passion alive.
MIX THINGS UP
New sexual positions, sexual acts you haven’t tried before, or don’t do very often, new lingerie and new locations are all great ways to maintain novelty in the bedroom.
DON’T STRESS ABOUT IT TOO MUCH!
Sexual dissatisfaction is a solvable issue. When you’re in the process of trying to regain the passion you once had, it’s best not to worry or obsess about it all. Furthermore, don’t worry about trying to reach some magic number; there is no amount of sex that is “normal.” Some couples need sex every day, other couples are happy with once a month. What matters is that both you and your partner feel satisfied.
SOMETIMES, COUNSELING HELPS
Counseling can be necessary, especially if there are other unresolved problems in your relationship that preclude you from having a loving sex life, such as unresolved arguments and resentments. Or, if you’ve already tried everything in the hopes that your partner will reciprocate, and he or she still isn’t willing to give you the sexual satisfaction you need, counseling may help you two sort things out.
You deserve to be happy.