One of the hardest aspects of dating in the modern era is figuring out the appropriate time to talk about STIs and safer sex. Most adults know they should have these conversations. That is why it can be so upsetting when you find out that the person you’re dating has an STI and didn’t tell you. It may make it a little less painful to find out that they thought they had a good reason not to tell you, even if that reason was wrong.

Telling someone you have an STI is hard, even if you know you should. Here are 10 reasons why people sometimes don’t tell their sexual partners when they have an STI. Some of them are understandable; others are not.

Often the person with the STI will avoid having any type of sexual encounter. They’ll postpone intimacy because they don’t want to put their partner at risk, and they want to put off having the difficult conversation.

Comprehensive STI testing isn’t part of routine medical care. That means that people need to actively seek out medical screening for STIs. It’s the only way for them to have an accurate idea of their status. Unfortunately, lots of times they don’t. Often, they don’t know they should.

It’s not just a problem for herpes. The lack of recognition that it’s important to inform a partner about risk is also frequently seen in skin diseases that are not primarily thought of as sexually transmitted, such as molluscum contagiosum, a condition caused by a poxvirus that spreads small, round, firm, often itchy bumps.

That’s why in situations in which only oral sex is on the table, many people don’t think that it’s necessary to disclose an STI. They don’t think it’s a danger, so why should they make the emotional investment of revealing an STI? That’s particularly true if they’re restricting themselves to oral sex because they’re only having casual sex. However, oral sex can spread a number of STIs.

On the other hand, a person can be so wrapped up in their own problems that it simply doesn’t occur to them that if they have an STI, their partner may have one, too. They are so focused on getting treated and moving on that they don’t notify their partners about getting tested and treated. They just hope that their symptoms will go away before anyone notices. Unsurprisingly, they often end up with recurrent STI infections.

It’s hard to figure out at what point in a relationship it’s appropriate to let your partner know you have an STI. Before you have sex seems like a clear boundary, but do you have to talk about it before you kiss for the first time? What if they kiss you? Should you bring it up on the first date or should you wait until you know there’s a possibility of developing a real relationship?

These are really difficult questions for people to answer, particularly for highly stigmatized STIs like HIV (human immunodeficiency virus) and herpes. If you’re still in the early stages of a relationship and wondering why your partner didn’t bring up the topic sooner, it might be worth giving them the benefit of the doubt.

Someone who tells you they were waiting for the right time may be telling you the truth. Telling someone you have an STI is a hard thing to do.

It’s hard to disclose an STI infection when you don’t know that the person you’re talking to is even interested in a sexual relationship. The very act of asking makes it easier. You’re telling a potential partner that you want them. You don’t have to worry that they’re presuming a relationship that doesn’t exist or moving into the realm of sex when all you’re thinking about is friendship.

If a person already knows they have an STI infection, this reason is similar to the others on the morally questionable scale. However, they may actually have put some thought into protecting you if they’re using this excuse.