Halliday & Matthiessen (2014: 501):
The special characteristic of the finite clauses is that, since these nouns are inherently ‘enhancing’ in sense, the circumstantial relation may, or may not, be restated within the clause: we may have either the day when/on which you came, with when, on signalling time, or simply the day (that) you came, with no indication of the temporal relation other than the Head noun day. In other words, the finite clauses are either like those of type (i) above or like elaborating clauses – that is, typical ‘defining relative’ clauses, except that they cannot take which without a preposition (you cannot say the day which you came). Examples:
I don’t see any particular reason [[ × why I should]]
This was the first occasion [[ × that I had to help in doing an experiment on a living man]] .
The only other place [[ × I would want to live ]] (is New Zealand)
The people downstairs – there’s no way [[ × they could have got out]] .
That’s the only reason [[ × I quit with Far Tortuga]] .
We shared a place in Italy the summer [[ × I was working on it]] .
All of these have four variants, two explicitly enhancing (e.g. the reason why/for which I like her) and two like elaborating (e.g. the reason (that) I like her).