I've noticed some people, especially younger ones, get into relationships that result from platonic friendships. Some turn out successful, others don't. The relationships that don't end up well are rushed and *sigh* involve someone not knowing what they want.
1. You're not physically attracted to each other.
If you're going to date anyone, it goes without saying that you must have the urge to get physical with them. Every time you're together, you should be craving them and teasing them a bit. You don't have to constantly kiss, cuddle, or handhold (which is annoying by the way), both of you just have to like what you see and think "Dayum." Any two people who aren't physically attracted to each other shouldn't be dating and expect a favorable outcome. Often times, they break up quickly due to the lack of sexual chemistry. Attraction is basic, something that shouldn't be forced and is deep-set in our biology. You can't merely be attracted to someone's personality and don't ever try to convince yourself you're attracted to someone when you're not. Don't waste their time or keep it to just being friends if you're not attracted in the first place!!! And if you decide to date and you're not having sex with each other yet after a month or so of dating, you've got some questioning to do.
2. You don't "click."
You've got to be able to laugh at each other's jokes, have similar interests, understand each other, and completely be yourself around them. If there's any part of you that says "This person could have a better, more interesting personality" or "This person is so boring to be around," you should pay attention to it. It's fine if you can accept their quirks, but if you're annoyed by them, you're most likely not going to be happy around them.
You know you've clicked with someone if your day gets better by seeing them and being around them, and still thinking about them after meeting up. Also, you may click with someone as a friend but that doesn't mean you'll click in a romantic sort of way. Maybe your pace is totally different from theirs.
3. You're unsure of what you want.
Don't jump into it if you're uncomfortable; listen to what your brain is telling you. Ask yourself this serious question: "Is it right for us to be more than friends?" If it is, go for it. If it's not, leave it be. If you're on the fence, leave it be. Save yourself or someone else from getting hurt. There may be someone better out there for you.
4. Your dating would cause drama (with other friends).
Chances are: if you're friends with your crush, you two belong in the same friend group.
There may be friends of yours or his/hers who get jealous and try to get in the way between you two. The person you're interested in may actually be interested in one of your friends. You should already be aware of any weird situation that's occurring.
Your friends may feel left out if you two ONLY concentrate on each other or bail a lot. Friends play an important part in your relationships, believe it or not. Your homies are always by your side and they've known you for a long time. So, your friends have an opinion and they know what you deserve. Their opinions matter, but that doesn't necessarily mean they know what's best for you. If the friend is the sole individual trying to cause a scene and not both your partner and friend, it's the friend's problem and you better tell them that the relationship is none of their business. That's life, even some friends can become jerks and if you're having an awesome relationship, you shouldn't let that bad friend get in the way.
5. He/She has baggage!
If your friend used to be an emo, is suicidal, still isn't over an ex, or is involved in a number of other related things (whatever it may be), it's a red flag! Don't get in that mess and let them solve their own issues. If you want to help, help but don't get sexually involved with them.
6. One of you is already stuck in the "friend-zone."
It's difficult to get out of the friend-zone and it's not even worth the effort to try getting out of it. If you're in the friend-zone, all you can do is accept it. Your friend made his/her intention clear and set the boundaries. You have to respect that or the friendship will end due to all the unnecessary chaos you might cause. CLEAR communication is key. If you're friendzoning him/her, do so in a respectful manner. Also, if someone changes their mind quickly about the friend-zone, that's just a sign the person is unsure of what they want and has some maturing to go through.