Why might a legislator vote for or against a bill?

The main influences of legistator voting are party influences and constituents. party influences are extremely strong, as party polarization is at an all-time high. so if you have a Democrat, that Democrat would typically not support a republican run bill. Also constituents, which are the people the legislator represents, also has influence over the vote due to the fact they are the ones voting in the legislator.

If the bill is stupid like stopping at railroad tracks even though it's not coming they might vote against it