Inverse/negative relationship between investments and interest rate The investments (I) have a negative relationship with interest rate (r)
$$
I = e-f \cdot r.
$$

So $e$ can be interpreted as a minimum level of investments while $f$ can be interpreted as how much the interest rate (r) affects the investments (I) negatively.

I don't understand why an increment in interest rate (r) implies a negative effect on the investments (I).

I thought that my investments would become more valuable if the interest rate increases, so from my point of view, people would be more interested in investing if the interest rate increases.

I guess I have misunderstood something.
The investment I earns income in the future. This income can be compared to the risk free rate of interest in the market to determine whether the income earned on I exceeds the opportunity cost of investing in I.

When interest rates rise, it is more worthwhile for an investor to purchase risk free bonds rather than invest in I. This decreases the demand for I, hence its price decreases.