If I am a resident in MA, and I worked only in NY and CA, why do I owe MA money?

MA taxes its residents on all their income, no matter where it is earned (so do other states with income taxes).

If you had to pay CA and NY taxes on the income you earned in those states, MA will give you a credit (against MA taxes) for what you paid to those states.

To get this credit, you have to prepare the NY and CA returns first, and then complete the MA state return. This way, the MA tax program will know  how much you paid to the other states (it transfers automatically in TurboTax).

Thank you for this information! But what if my MA income is $0 [which it is: $0 in MA and non-$0 in NY (student job) /CA (internship)]? According to the MA website, "Full year residents who receive or accrue Massachusetts gross income in excess of $8,000 during the taxable year are required to file a Massachusetts income tax return. Even if they calculate a zero tax, the requirement to file still exists. Full year residents file Massachusetts Form 1 - Massachusetts Individual Income Tax Return." - found at <a rel="nofollow" target="_blank" href="http://www.mass.gov/dor/individuals/filing-and-payment-information/guide-to-personal-income-tax/filing-requirements/who-must-file.html#Res">http://www.mass.gov/dor/individuals/filing-and-payment-information/guide-to-personal-income-tax/filing-requirements/who-must-file.html#Res</a> . From this information, it appears that in my situation as a full time resident but with income less than $8,000 that I do not need to file a state return.

That would be the case. Often, however, taxpayers file returns even though they are not required to. There are many reasons, mostly to take advantage of certain tax credits or get refunds of tax withheld. Some do it just for year-to-year consistency. If you are not required to file a MA return, you don't have to.

As it is, you will probably get refunds of any tax withheld by NY and CA.

Awesome. Thanks for all the help!

Glad I  could be helpful!