From the point of view of cosmology we measure a very small but non-zero cosmological constant and want it to be this value we measured.
From the point of view of quantum field theory, however, it is very unnatural that the cosmological constant is non-zero and very small. The question is similar to the matter-antimatter asymmetry: In the early universe we had a lot of matter and a lot of antimatter, about the same amount of each but not exactly. Then matter and antimatter annihilated but due to the very small difference in amount some matter remained which is all the matter which we observe today. So we could ask: Why were the amounts of matter and antimatter so similar but still not exactly equal?
Similarly, there are hypotheses that the small value of the cosmological constant emerges due to zero point energies of several fields (which individually are very big, rough estimates give over 120 orders of magnitude compared to the value of the cosmological constant) almost cancelling out, but not exactly, leaving a small difference which is the cosmological constant we measure. This is the so called naturalness problem: It seems like an unnatural coincidence that two or several very big values add up to a very small but non-zero value.