Why Does LTSpice Always Show Negative Voltages in a Rectifier Circuit?
To answer your follow-up question Jim, if you move the ground to Vrin2, Vrin1 will be a sinewave at 0v dc offset. But there is another way to see the sinewave, and that is with mathematical plotting.


Right-click the chart, choose "Add Trace".
In "Expression(s) to add" type in: V(vrin1)-V(vrin2)
Shorthand: click V(vrin1), press -, click V(vrin2)


Note that the voltage source is ideal by default, so has no resistance, thus will produce a perfect sine wave. Expect real-world measurements to never match simulation. So here is added 1Ω of series resistance, and a capacitor chosen with real-world parasistic values (a good 25v Nichicon):