Sometimes, when you've been single for a while, you're not supposed to look for a new partner just yet, as sometimes a relationship just isn't what you miss in your life. You might want to work on yourself before you go looking for a new partner.
You have not yet learned to love yourself
It might sound cliche but you should not be dating anyone until you have learned to be satisfied with who you are. If you're unsure about yourself, chances are you think your partner is cheating on you or lying in general. These feelings might occur solely because you are not happy with yourself and you can not imagine that he prefers to be with you over all the other girls. Therefore it is important to only start a relationship if you're happy with yourself.
You're busy enough as it is
If you are too busy with your studies, work full time or have lost a lot of time taking care of a family member, it might be better to stay single for a while. If you already have too many things going on in your life, it may very well be that a relationship will only stress you out even more. 
You still have feelings for your ex
If you still have feelings for your ex, you're not ready to start a new relationship. It is not fair to the person with whom you begin dating, and it isn't fair to yourself. You have to give yourself time to get over your ex before you start again with someone else.
You're not sure if you want to start dating 
You should not start dating for the wrong reasons. If there are no boys that you really find interesting, you should not be dating. It is best to wait until you find someone you really like. Otherwise, you're much happier without a partner.
You do not want a serious relationship, but just one night stands
If you do not want to be in a serious relationship, do not try to have one. If the person you date just wants a serious relationship, you can not lie to him and deceive him like that. Are you just looking for one night stands? That's fine too, as long as you let your partner know about your intentions right from the start. Be honest and upfront about it. 
You don't want to put effort into a relationship 
If you do not want to take the time to meet his parents and meet up every weekend with him, you shouldn't be in a relationship yet. If you are not willing to do the things you should do for a healthy relationship, you should stay single!
You enjoy your life as a single
Being single is not nearly as bad as some people make it seem. If you are single, you have a lot of freedom. You can do whatever you want on the weekend, you have enough time to meet up with your friends and you have the freedom to flirt with others. If you enjoy your life as a single, don't feel forced start a relationship with someone. Just enjoy yourself!