It does depend on your other options, but people are very bad at estimating the value of their other options. They idealize potential partners they don't know well, they overestimate others' interest in them ("She looked at me for slightly longer than she looked at him! That must mean something!"), and so on. An "outside view" estimate based on either (a) what it was like to be single or (b) your previous long-term background including past relationships is less vulnerable to bias, and likely to be more accurate.

Breaking up in order to enter a different relationship is also socially undesirable. For instance, people will see the new partner as having "broken up" your old relationship (noted upthread to be a dick move), which will damage their social status.