Do I have to claim a capital loss on 1040 line 13 If I had a loss?

and why would you not claim it

No, you can't just skip a year. If the last line on the 1040 before the personal exemptions drops below $0, you won't use up the entire $3000 when calculating the 2017 carry forward.

You do if you want the tax benefit of taking it.

If line 14 shows a carryover from last year and line 21 shows a loss of $0 for this year, it will be treated as an error in most cases and the IRS will simply recalculate using as much of the $3,000 loss for 2016 as your income allows. Too bad if it does not reduce your tax -- those are the breaks.

If lines 14 and 21 are both $0, there won't be any gain to carry forward to future years at all. The IRS *might* catch this type of error but as it goes from one year to the next, that is not always the case. If they don't and you then try to pick up a carryover next year, this year shows $0 so it would be disallowed as they DO look back to see if there was a loss that would have been carried forward the prior year.

You must claim $3000 each year.

NO you cannot skip years as it fits you best. Once you stop claiming the $3k maximum, there is no picking back up and reclaiming later on. Those unclaimed losses are.....well LOST.

Use it or lose it.

Technically, you don't have to claim it but you can't carry to the next year either.

You can't just save it for a year where it helps you if your income is low this year.