For better or worse, relationships change over time. Over the long term, things can erode slowly and a healthy relationship can be unhealthy without you even realizing it. Some things can help, like being compatible. Truly compatible and willing to compromise.

"Compatibility is one of those things that you need on some level to make it through the long haul," Matchmaker and dating coach Karenna Alexander tells Bustle. "Having compatibility when it comes to inner qualities — like one's values — is the most important thing. Hobbies and surface characteristics — like vacation preferences or decor preferences — are less important. However, what is more important than compatibility is emotional maturity and ability to compromise." That's what's going to help you out in the long run.

"Even if you start off very compatible, things change throughout the relationship," she says. "You are always having to make compromises, no matter how compatible you are with someone. So I think one's ability to compromise and one's emotional maturity are better predictors of whether a relationship will make it."

If you don't have an inherent compatibility — and the ability to change and compromise so you stay compatible — things can take turn. But, because every couple is different, there are a lot different ways relationships can become unhealthy. Here are some of the ways that things can shift over time, because resentment takes a while to simmer.

1 You Stop Making An Effort

There's the "we haven't quite fallen out of love with each other but we're not really in love" place that so many couples get to. You take each other for granted, treat each other like roommates, and have lost the spark. Make sure you keep your love alive by still flirting like you just met. "Couples who stop flirting are couples who stop anticipating," Certified Relationship Coach Chris Armstrong tells Bustle. "Things go blasé and what was once an unpredictable stroll is now an expected lull."

2 One Person Becomes Resentful

There's often a dominant person in a relationship. But if it's always about one person's needs, then the other can only last so long before they get resentful. "Not asking for what you want in a relationship can make you have much less pleasure and joy, and eventually stop thriving in that relationship," Marthe Schneider, co-founder and co-developer of Authentic Tantra, tells Bustle. "From there, resentment, disconnection, and misunderstanding grow." And then you've got an unhealthy, toxic relationship.

3 You Turn Into Squabblers

If you don't know how to have a constructive fight, issues will start to bubble up in weird ways. "The one thing you should look for in a relationship is good conflict-resolution skills," Dr. Fran Walfish, Beverly Hills child, parenting, and relationship psychotherapist tells Bustle. "People are generally great at communicating the warm and fuzzy stuff. But when it comes to working out the kinks and differences of opinion that lead to unpleasant fighting, most folks gets stuck."

If you don't know how to do it, you may just end up nit-picking and annoyed with each other all the time, which isn't good for either of you.

4 Codependency Sets In

Maybe things are great and you're getting along. Maybe you're always getting along. In fact, maybe you're getting along so well you never do anything without the other. If you find that you depend on each other for validation, decisions, and every little thing, you've probably become too codependent.

"It’s very important to have independence in a relationship," relationship etiquette expert Mara Opperman tells Bustle. "Successful, healthy relationships allow for the both people to form a bond which lets them to not only grow together but also to grow independently as people. It’s essential to have your own sense of autonomy while feeling you can depend on each other. Also, if you give up your independence and abandon the things that used to make you happy, it will be reflected in your relationship."

5 One Person Becomes Controlling

One person is often more dominant, but if they're jealous or just controlling, that's something that can slowly get out of hand over a long-term relationship. So if you or your partner has a tendency to need to be in control, make sure that you're keeping an eye on it and keeping it in check.

6 Your Sex Life Fades

If you both start to lose your sex drive over time and you're happy having less sex, that's totally OK — and perfectly normal too. But if you've just stopped making the effort and one of you is feeling unwanted, that can turn into a toxic situation very quickly. Make sure that you're still communicating and making sure you're both feeling OK about your current level of sex.

“Keeping love and sex alive in your relationship is what keeps the relationship alive,” Tina B. Tessina, Ph.D. (aka "Dr. Romance"), psychotherapist and author of How to be Happy Partners: Working it out Together, tells Bustle. “It’s like the roots that feed the tree. To keep that vital energy going, and the sap rising, you need to provide something new and interesting. Seduction can be as simple as causing your partner to ask what you’ve been doing that has you so energized and interested. When you’re enthusiastic, you’re seductive— it’s the most attractive we can be.”

7 There's Borderline Unfaithfulness

Obviously if one person is cheating then it's not a healthy relationship, but it's not always so clear cut. Sometimes in a long-term relationship, one person is keeping an eye out or forming borderline inappropriate relationships. "People are far pickier now because of the apparent availability of new attractive partners with online dating and mobile apps," Dr. Peter K. Jonason, researcher at Western Sydney University, tells Bustle. Maybe it's just talking to someone online, maybe it's having a too flirty relationship with someone at work. But sometimes being in a relationship for a long time means that a healthy flirt turns into something more.

There are tons of ways to have a happy, healthy long-term relationship — so don't think you're doomed. But people do change as time goes on, so it's just a reminder to touch base, keep communication open, and stretch that self-awareness muscle.