The point of science is to create a single theory by which we can explain everything.
As this is too difficult, we take things in chunks.
To be accepted as a scientific fact a hypothesis or theory must be proven correct in repeatable experiments.
Physics is easiest to prove right or wrong (well, classical physics) for example: SUVAT equations such as : S= ((u+v)/2)t can be proven correct by experiment repeatedly. This means we can accept this equation as correct.
Chemistry is a little more difficult to prove- if I can't see pi bonds forming, how do I know that they aren't just three sigma bonds? For things like this we have theories that EXPLAIN observations, but ultimately we could be totally wrong.
Quantum Physics gets a little more tricky, especially with the whole wave/ particle duality as shown through electron diffraction grating (how can a single object pass through two seperate slits simultaneously whilst NOT being observed yet through only one when observed?)
Biology is less 'scientific' still, because as someone already said- it is hard to replicate conditions for repeat experiments.
Psychology is less scientific still, yet still scientific in its approach.
Think of it like this: Maths is core. Physics is applied maths mixed with a touch of philosophy. Chemistry is applied physics. Biology is applied chemistry with a bit of electricity. Psychology is applied biology.
So yes psychology is science: just watered down- not due to flawed theories, but flawed base science (i.e everyone is different).
Thing is, it doesn't really matter so long as you have a passion for what you study
